NPCs cannot intimidate PCs in 5e, so it's rather moot point. I don't actually recall if this was possible in 4e.
It's not really very clear. My own view is that
intimidation in combat is an action like anything else, perhaps dealing psychic damage and/or an effect like being pushed (ie recoiling in fear) - the best version of this I recall is not based around the intimidation skill at all but rather is an attack power (Horrific Visage) on the Deathlock Wight. Whereas the power vs skill distinction is important for PCs (because of how it feeds into action resolution) it doesn't matter very much for NPCs/monsters (for the same sort of reason as PbtA games don't need monster stats comparable to players' playbooks).
Out of combat, intimidation by a NPC (again somewhat as you might think of it in a PbtA context) is a GM move in the fiction, whether soft (typically as part of framing) or hard (narrating a consequence in a skill challenge). Because the rules are vague and the traditions are strong, I think it's an area where most GMs will benefit by treading lightly - I can't think of an intimidation example straight away, but I do remember that in a skill challenge where the PCs were negotiating with a Pact Hag (who has many verbal manipultion-oriented abilities) at one point I narrated the fighter PC taking a step to where the Hag wanted him, so she could pull the cord that opened the trap door. The player did not have an issue with this in play; but when I posted about it on ENworld it did cause some outrage!
This actually shows what I'm saying. The 4e character that doesn't spend any effort on a skill is at the same chance to succeed at an easy take at 1st and at 30th. They have nearly no chance to succeed at any other tasks at 30th, but can at 1st. Meanwhile, the 4e character that specializes in a skill succeeds at a hard task roughly at 50/50 at 1st, and, given your best bonus above, about 50/50 at a hard task at 30th.
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And while I know you weren't making any points about 5e, the reality is that the above holds true for the unskilled player -- you're about 50/50 at 1st for an easy task and about 50/50 at 20th for an easy task. DCs in 5e are mostly set by approach and the challenge according to the rules of the game (published adventures ignore this, which I find maddening). So it holds up. Meanwhile a proficient character goes from a +2 to a +6, but the DC range never changes, so they actually see improvement against relevant challenges.
that 4e character faces a constant treadmill of increasing DCs, so their actual chance of success is the same as the 5e character, who doesn't just add numbers to their sheet to keep up with the rising DCs. A 4e character, your wizard, at 1st level, with a 0 stat and no proficiency in athletics, faces a DC 8 for an easy athletics challenge. That's a 65% chance of success! The same 5e wizard faces a DC 10, for a 55% chance of success. Now zoom to 20th. No build resources are put into either. The 4e wizard has picked up +10 for half level, the 5e character has not improved. They both now face an easy task. The 4e character's DC is 18, meaning they have a 65% chance of success. The 5e character's DC is 10, which has a 55% chance of success. Wait, neither actually improved!
But, let's look at a medium DC. At 1st, the 4e character faces a DC 12, for a 45% chance of success, and the 5e character faces a 15, for a 30% chance of success. At 20th, the 4e character faces a DC 25, and has a 30% chance of success. The 5e character faces a DC 15, for the same 30% chance of success. Huh. 4e lost a step.
Now, hard. DC 19 vs 20 at 1st, so a 5% difference with advantage to 4e. At 20th, the 5e character still only has a 5% chance of success, but the 4e character cannot succeed at all with a +10 vs a DC of 34.
This is the straight skill system. It's not like 4e characters actually improve at the things that they are actually doing.
I'm not sure I fully follow your comparisons, because you seem to be equating Easy/Medium/Hard in 4e to Easy/Medium/Hard in 5e, but I don't think that can be right.
In 4e those difficulty judgements are (in some slightly obscure fashion) level relative - the way I personally tend to resolve that obscurity is by treating Medium as the level-appropriate default (be that
astral teflon slime or whatever other setting element is generating the DC) and then treat Easy or Hard as reflecting situational adjustments that aren't tied to the particular setting element. But that's my own approach and I can't say I've always followed it utterly consistently. (The Essentials-era skill challenge rules mostly solve the problem by putting the difficulties into the skill challenge structure, which brings it even closer to HeroWars/Quest or Cortex+ Heroic Doom Pool-style resolution.)
Whereas in 5e, I would have thought those difficulty judgements are largely "objective", similar to obstacles in BW or the throws needed for success in Classic Traveller or all the prescribed DCs found in 3E D&D.
But putting that to one side, don't your numbers show that for 4e PCs, the higher the level the harder for the less-focused PC to do well compared to the more-focused, as we move from Easy to Hard; and likewise for 5e PCs, the higher the level the harder for the less-focused PC to do well compared to the more-focused, as we move from Easy to Hard. In 4e this is because Medium and Hard DCs grow at more than the level-bonus rate; in 5e this is because only a focused PC has a growing bonus on the check. So (subject to my comments above about what the difficulties mean in the different systems) I'm not sure I'm seeing the difference that you do.
ignoring the one character above that has check values in the 40s for a set of skills, which is apparently due to some massive outside the PC bonus, I'm guessing, it looks like non-outside pumped scores top at around 30
Kinda.
Stat bonuses tend to max out around +8 to +10 (ie 26 to 30 - people will tell you that a PC with a 16 in his/her prime stat is hosed but that's not true: but Derrik is a fighter with 16 starting STR and so 26 at 28th (the last boost) and is extremely effective - of course a careless build with a lower stat might suck, but part of the point of 4e is avoiding careless builds!). With +20 for proficiency and level that's +28 to + 30. With an epic-tier item granting +6, that's +34 to +36. Another +2 from race or theme or feat is not atypical. So those top bonuses tend to be high 20s to mid 30s, depending on all the elements in the mix. If you look at my chart you'll see the physically-focused Derrik (fighter); the socially-focused Jett and Tillen (sorcerer and paladin; Jett also has reasonable Acro and Stealth as he is a secondary DEX sorcerer and until he lost the ability was a Cloud of Darkness-using Drow); and the archer ranger Ravian (with the typical skills you'd expect - Nature, Perception, Acro - no Steath because he's a hybrid cleric/ranger and so gets only 3 skills by default - from memory the training in Religion, which sits on the lowest possible base, is the result of a paladin multi-class feat).
The most interesting is the "skill monkey" Malstaph, and invoker/wizard who is about as close as you can get in 4e to a non-combat character (which is not to say he sucks in combat - he's not bad given 1x/enc AoE blind and AoE domination). The 40s are the result of his epic destiny, Sage of Ages, which gives +6 in the five knowledge skills (Arcana, History, Religion, Dungeoneering, Nature). Most epic destinies gives +2 to one or two stats, but the Sage of Ages doesn't, granting this big skill bonus instead. It's an interesting design attempt but I think in the end it doesn't work - it's a good fit for certain particular abilities (a feature of the destiny itself, and certain rituals that look for degree of success on an appropriate check), but puts a bit too much pressure on the skill challenge maths. Probably +2 to one of INT or WIS and advantage on those checks would be better, with the destiny feature being appropriately scaled down and the rituals just falling where they lie as a result.