That's a weird argument. And not only because of the data-less assertion of what other people are going to assume. Or because in my experience it's the good players who sometimes like to experiment with poorly constructed characters.*
What assumption? That D&D is built around a team concept of play? Is this something that people have a different opinion on? I mean, sure, you can play a different way although I haven't seen any D&D play that actually does so and focuses on independent characters outside of maybe solo or pair play, and even there henchmen/retainers/followers are a big deal. No, D&D is clearly built on the team concept of challenge. Class design, niche enrichment, encounter design, the resources game, all of this is pointed directly at team play. Every published modules I've ever seen is designed based on team play. I'm not sure this is either data-less or even really an assumption. It's pretty clear.
Rather, it's a weird argument because this is a debate about the relative impact of char construction vs. skill at play, not other people's assumptions.
Again, the fact that D&D is a team sport is neither an assumption (as presented) nor is it irrelevant to the discussion. Yes, we're having a sidebar on the distinction between player skill and character build, but I distinctly noted at the start of the paragraph I introduced team play as going back to the OP and that question -- ie, it's addressing a wider topic than the narrow subtopic we'd been focusing on.
*Which I was thinking of as, say, the 13 Str fighter with high Cha and Int, or the Str-based rogue, etc. Not sure how we got into 8 Str fighters.
If the role in the party that you're taking is that of the primary melee combatant, then this is still a build that will be much less successful at that role regardless of smart play. You can't beat the numbers, here. High CHA and INT are not things a fighter can really leverage very well. Sure, if you go EK, higher INT works, but limited spell slots compared to other casters means the higher DCs or attack bonuses are much less useful overall and will show this. Or you can pick the BM abilities that use INT for DCs (I don't recall if any use CHA, perhaps). So, as a fighter, focus in these two stats is of limited return, while not having a good STR is very limited (assuming this isn't a stealth DEX build, of course, so low DEX as well). The fighter has limited proficiencies to even take advantage of these stats, relying entirely on background to gain up to 2 skills that pair well. So, yes, there's some utility here that can be achieved by trying to go off class.
But, a wizard doing the same for INT does as well in those few areas the fighter can claw out AND is far more competent in their intended shtick as well. As is a Bard or Sorcerer for CHA. They at a minimum tie the fighter in the narrow places the fighter can leverage those stats AND retain full capability in their primary roles because those stats are prime stats for them.
The STR rogue is interesting, but ultimately you're either discarding a primary class ability (sneak attack) to do this for compensatory benefit (any benefits are to small things like STR(athletics) checks or raw strength test, certainly not on par with sneak attack) or it's kinda a gimmick where you're still using a finesse weapon, just with strength, and that doesn't look any different on attack rolls. If you really go for it, it pull a powerful ability for no compensation, which will absolutely be felt (the strong rouge is less capable in combat?), or it's just moving numbers around and not actually a hinderance mechanically.
Look, I don't think we're actually disagreeing that it would be preferable to still have a great character that can be well played by going off type in build, but, sadly, 5e isn't the game that supports this. Some minimal need to at least address well your core shtick is built into the game, and built into the expectation of the play of the game. Showing up with a gimped character is quite often not going to be edgy and cool, it's just going to be selfish because you're not going to be able to fill the role the group expects. A GM could work around this, yes, but, again, that's not about you're skill but the GM making allowances. Skill is not sufficient to shore up bad character builds, and quite often trick builds are not really that great, either. I'm not saying everyone has to optimize -- heavens that would be a drag -- but you also can't assume that the game supports your no STR no DEX fighter who's trying to get by with a keen mind and a silver tongue. Honestly, if you want to play that character, you absolutely can -- play a rogue or a bard. Lack of a primary attack stat will still hurt, but the class allows for more abilities you can leverage.
