Now, you can do what 4e did and add your level or half your level to everything, but that doesn't actually change the math for equal level creatures. You'll still suck at what you suck at.
In 5e, it'd be your proficiency bonus, without the +2, so +0 to +4 over 20 levels. Enough for you to sorta tread water vs increasing save DCs (you'll still fall behind, but by one or two instead of around six).
It just makes lower level creatures not a threat and higher level creatures overwhelming.
Not when scaled to Bounded Accuracy's almost moot degree of advancement. It won't eclipse even the lowest save DCs for characters with really bad saves (like a -1 dump stat you never improve), at 20th, you'd still fail a DC 10 on a natural 6, a more typical first-level-monster DC of 13 is still going to overcome your 20th level PC almost half the time. I don't know how much more vulnerable you'd want to be...
Heh. The classic game had 6 saves, not by stat but by category, and not without ambiguity (including various things calling for a save vs categories not listed in the DMG). Using all 6 stats is a nice incentive against dump stats and for more all-'round sorts of characters. Just not enough of one.
Depends how you measure "most helpful". For example, if you fail your saves only on a 1 (95% success), advantage cuts your failure rate by 95%. If you fail 50% of the time, it cuts your failure rate by only 50%. ...On the other hand, if you succeed only on a twenty, advantage doubles your success rate, which can e.g. halve the duration of enemy spells on you.
Statistical sleight of hand. Yes, nearly doubling your rate of success sounds better than merely increasing it by half. But, the reality is that if you have advantage an need a 20 to succeed, you'll fail 90% of the time instead of 95%, the equivalent of gaining a +1 to the roll, while, if you are looking at a 50/50 shot, you'll by succeeding 75% of the time, the equivalent of a +5.
+5 is better.
Advantage/Disadvantage has the greatest impact when your chances are close to 50/50.
I'd argue that it's actually fallacious to interpret advantage/disadvantage as a linear bonus on a d20. Advantage isn't "equivalent" to +4 on your roll, nor is disadvantage equivalent to -4. Both of them are actually ways of transforming the probability distribution into something roughly bell-curved.
The equivalents are perfectly valid, and very helpful for making the point to gamers used to comparing them.
A +5 on a d20 when you needed an 11 is going from a 50% to a 75% chance, exactly the same% as having Advantage, bell-curve be damned. And, it's not a bell-curve, anyway.
In 5E fights, it's usually more advantageous to impose disadvantage on an enemy than it is to gain advantage yourself. In fact, there are even cases (e.g. archery duels) where it can be a smart move to impose disadvantage on both parties
Sure. It depends on who is more greatly impacted by the mechanic. PCs actually tend to hit around 65%, but how often they're hit varies more depending on class, build, & gear. The combatant whose roll is closer to 50/50 will be more greatly impacted by either advantage or disadvantage.
I presume we're talking about the common 5E idiom in which one successful save removes the spell and puts you back in play. In this case, you're best-off helping the guy with the weakest save, within reason, in order to reduce the effective duration of the spell.
You're making several questionable assumptions, there. One is that the combat goes on long enough for the difference between being out for a long time vs a very long time matters. 5e combats tend to be short. Another is that you're applying a bell-curve average that's valid over many, many combats, to a decision that happens in a single combat. Now, yes, over a character's entire career, that kind of reasoning holds. But, that's not how people made decisions, and the degree to which it's better won't really be experienced at the table, by anyone.
Now, if Advantage were an optional re-roll, take the second roll - or even 'roll two different color dice, designating one as the normal die, one as the advantage die,' it might, what you're talking about might be more evident to people. As it is, the tendency is to look at advantage having 'helped' you whenever one die succeeds and the other fails - which overstates it's apparent value across the board. With an alternate method by which you could note what you 'would have rolled without advantage' it becomes more apparent that "half the time" advantaged is 'wasted' on a 50/50 roll, because you 'would have succeeded anyway.' On a series of saves, that makes the point you're making apparent.
However, if you're just trying to maximize the chance that, on the next round, one of your locked-down allies gets to act, going with the 50/50 guy is the best bet.
And you're not making an entirely invalid point. If you're trying to maximize the total number of round available to your allies with no particular time horizon or preference for when they take those rounds. That's just not what most people are trying to do. Because combats are short. Because advantage looks, in play, even better than it is. And because people simply don't make decisions that way.
It really isn't possible to generalize a specific point on the d20 where advantage is "most helpful." It depends on the details of the situation and what the save is being made for.
For certain more nuanced and farther-looking definitions of 'most helpful,' perhaps. But the + equivalent is entirely valid. In a situation where a +5 to a roll with a 50/50 chance of success wouldn't be very helpful, and a +1 to a different 95/5 roll would be more helpful, sure, advantage wouldn't be very helpful, either.