No that is absolutely not true because the advantage on 4 skills (5 at 10th level) and the expertise on four tools comes from the Rune Knight subclass explicitly, not from the feats.
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Yeah, Rune Knight does enable this build better with all its advantages. But a significant part is that you've enabled the build with stat allocation and feats. Advantage multiplies with your other build decisions to make it worthwhile: getting advantage on a DC20 check with +7 is significantly different to rolling with +0 (64% vs 10%). You've played in to those class features with your feats and skills; but for a character that builds a more combat-focussed RK those advantages end up being close to ribbon features.
An an aside, I've rarely had tool use (other than theives tools) come up in my games. Different DM styles I suppose.
But there is a huge difference between being awesome at the social pillar (like the RK I posted) and able to "only really contribute" to the combat pillar. My post was a specific reply to someone who claimed that the class could not contribute. What I posted was a build that is dominant.
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The advantage is something that comes with the Rune Knight subclass. Slight of Hand and Deception advantage for example come with Cloud Rune, which is a pretty powerful combat Rune in its own right. Those things are part of the subclass design. The build I posted above specifically and purposely plays on these things which come from the subclass.
You've provided a counterexample, which is great, it seems like a neat build, but its definitely an exception to all the other possible configurations of fighter. Like you said, RK is way better at this than other fighter subclasses, but it also needs the investment of stats and feats.
But lots of people want to play a non-RK fighter.
Moreover the example above did not dump Dexterity. I had a 16 Dexterity. I think Sorcerer example is a poor one, because even if you took feats like the RK did you would not be as good. Even with a 20 Charisma you would not even be as good at the social pillar alone unless you really drove your spell selections for that specifically (Charm Person, Suggestion, Detect Thoughts etc). Would you be "able to contribute"? Sure you would.
I get it, this build with expertise and advantage in everything is REALLY good at doing things. Maybe even the best.
I think a lot of people's expections of 'good' is a little lower than 'dominant'. My last Sorcerer had +5 CHA, proficiency in 3 social skills, Lucky, a few helpful spells, and this was more than enough to be the 'face' of my party. He could have technically been better with Expertise, but this was rarely necessary. He was
good at those skills without sacrificing any part of his build.
I don't think people really want a non-magic flavored fighter that can do "great things"
This really depends on what you mean by 'great things', and
this is the heart of the issue for me.
A non-magical fighter shouldn't be able to fireball and teleport.
But currently most of them can't do anything more than auto-attack.
There's a happy medium somewhere in between those two points that WotC seems adamant not to explore, even though it seems to be highly desired by the community.
People I play with love the flavor of Echo Knight and Rune Knight. A subclass that did those things without being about Giants, or about a phantom going around but scored the same kind of damage, skill and mobility bonuses would not be as popular I don't think.
The flavour of both these subclasses is cool. I also like the flavour of Champions and Cavaliers, but unfortunately their rules are lacking.
There's room in the game for 'mundane' flavoured characters, that do not the same things but things of equivalent impact. For the people who want to play Achilles or Boromir or Lan Mandragoran, but are unsatisfied with only having 'I hit it with my sword' as an available action. From what I am seeing, having more mechanically intersting martial classes is one of the most common requests, homebrews, and additions to wannabe 5e competitors.
Well a Scimitar is not the best finesse weapon. If you look at a Rapier and Dueling you are doing 12-26 damage with your stick plus 1d6 when using Giant's might. A baseline EB Warlock with a 20 Charisma and agonizing blast is doing 12-30 which is about 10% better. That is not including the Fire Rune.
Apolgies, I used a Rapier in my calc but wrote Scimitar.
An important missing factor in your damage ranges is that the lower bound is 0 if you miss - which since you're not increasing DEX is more likely, so we can't just discount that.
The maths is long and not really the point here, but the answer ends up being that the non-combat-focussed Rune Knight is somewhere between 35% less DPR than a combat-optimised Rune Knight in the best case (fighting AC19) and up to 50% less as enemy AC goes up OR down. To me, thats not competent. YMMV with your party though.
All that is kinda tangents though. The main thing I was trying to say is that
a) Rune knight is probably the best fighter chassis to start with even for combat focus, except maybe Echo Knight
b) Saying that 'This one Rune Knight build can do skills' isn't a helpful answer to the general complaint that Fighters in general can't do skills.