How is discrepancy in optimization a problem, outside of jerk behavior?
There's the en passant steamrollering of someone else's niche-toes, which will bother some people more than others, and be more of a problem in some rulesets than others, but it's real.
There's the old "what challenges the one character might wipe everyone else" thing--the people talking about Scion were very clearly talking about this.
It's hard to keep all the players engaged and involved when one of them brought Superman and another brought Aquaman ...
I've seen all of those, in one form or another, over the years. I've been on both sides--sometimes I really knew a system, sometimes I really didn't. No pearl-clutching, here, just real-life experiences I can point to.
Okay but where in the core books of any TTRPG does it say all the players have to be the same? Sure, the same level, but is it required that all the PCs are "balanced"? If so, without mentioning level, define how the PCs must be "balanced"?
If one player chooses a more proficient collection of skills, feats, traits, powers, whatever than the other players - BUT ALL THE PLAYERS HAVE ACCESS TO THE SAME RULES - how is the optimizer crossing a line?
This just looks ... it looks like people who aren't "good at the game" want people who are to dumb-down their characters and that completely defeats the purpose of getting "good at the game". Look at other games: No one is telling Magnus Carlsen to stop studying and playing chess at a high level. No one tells Steph Curry to stop shooting so many 3s. No. Instead those great players are applauded and even worshipped because of their dedication to getting "good at the game".
Why can't we have that with tabletop RPGs? Because they're social? MOST if not ALL games are social in nature (at least the ones that require groups). What about Steve? Steve's the best player on his darts team. No one on the team told Steve "Hey mate: you need to stop being so good at darts". WHY? Because Steve being a darts-master means his team has a better chance of success.
But saying it's "one PC outshining others" isn't it because the players aren't competing, right? Saying optimization "imbalanced encounters" isn't it because most TTRPGs have specific rules for addressing PCs in the same party having different levels of capability (D&D does, GURPS does, a lot of superhero ttrpgs do), right? Saying it "defeats niche protection" isn't it because - again - most TTRPGs don't even mention the concept of "niche protection", right?
I mean if during Session 0 the group decides each player will play a specific role/class, IF one of them decides to make a character whose role overlaps into another character's, that is NOT an optimization problem: that is a PLAYER problem that should be addressed by the GM. Everyone that optimizes isn't a jerk, regardless of your limited experiences.
Could this be about jealousy? Because that's a possible explanation, right? I've been on a lot of teams and in a lot of groups where one or two people were just BETTER at what the group/team was doing than their teammates - and there was jealousy. Often.