I think there are 2 things that are true in relation to this statement.
1) The market as a whole is moving steadily away from "character building as a method of demonstrating/utliizing system mastery" and towards "character building as expression of style and aesthetic."
Is this part true?
I think I very much agree with the first part. But, as I see it, "the market" is growing toward "don't worry about it, its just a casual past-time". Which is completely cool. I'm not knocking it in the least. But I think that even within the relatively niche population of "people who play TTRPGs", the subgroup of "People who create and use accounts on TTRPG websites" is distinct and not terribly representative.
To use WOW as a rough example, it was common to have people who tended to prefer playing a Mage and people who tended to prefer playing a Warrior, and yet these same people would openly mock "RP" servers and have zero interest in character-acting tabletop RPG experiences.
I believe that the BOOM 5E is seeing is somewhere in between WOW and an ENWORLDer. They want the aesthetic of their character for sure. But only in a very casual sense. I don't think "building" is the right word. I'm not saying the people you describe don't exist. A lot of them exist. But they have existed for some time. I don't get the sense that there has been any shift there. I think system mastery is stable to slightly decreasing. I think character creation aesthetic is flat. And I think casual "I'm the wizard, let's go" is growing big time.
Your comment does make me think of the old 4E quote from Andy Collins:
"In a lot of editions of the game, classes compared to new classes were designed by [first] imagining what could exist in the D&D world, and now I assign the mechanics that make that feel realistic and then I’m done. Well the problem with that is, that you get an interesting simulation of a D&D world but not necessarily a compelling game play experience. A lot of the classes designed in the last 30 years are not interesting, are not compelling either in a fight or maybe out of a fight, but just pale compared to other characters on the table top.
...
So whenever we were approaching a new class we had to home in on what makes this guy special and unique within in the game - not just in the world of D&D but, since we’re playing a game, why is this game piece different than another game piece and why do I want to play it instead another game piece."
That certainly suggest 4E didn't lean into the aesthetic side of things. Not sure if that is really relevant to today's conversation. But the parallel (with kinda reverse polarity) of your comment drew that old item to my mind quickly. I guess I digress..
2) The Pathfinder player base has a disproportionate amount of the players who do favor character building as its own subgame, and that remnant is still large enough to be worth targeting.
I think this part is true.
I think PF2 might have tried to split the difference and ended up not squarely hitting the target for either camp.
There seems to be a death of a thousands cuts. You can talk to 5 people who decided they didn't like PF2E and get 6 different reasons why not.