That's the crux of the whole matter for me. I'm sure the game is fine without multiclassing or feats, but I'm going to be wary of a group that insists we play it that way, so I'll probably never have that experience.
If somehow one of my established groups suddenly collectively turned against feats and multiclassing that would surprise me, but it's not something likely to sour me on playing with them. I don't care that much about those things, and I certainly have character ideas that don't require them I'm happy to use for the next campaign.
But if I was joining some new, unfamiliar group and was told "we don't use those 'optional' rules" (people opposed to using them usually feel compelled to remind you that they are optional, as if D&D has any mandatory elements) that would give me pause. Firstly, because I personally would be advocating for allowing these player options so maybe we're not on the same page or maybe they're going to ignore my input. Secondly, because I think the reasons for not using these options are often bad, having to do with overbroad solutions to specific problems, someone trying to impose their idiosyncratic vision of what the game should be on how other people play, or a misguided belief that, stripped of all its "options", 5e is suddenly a carefully-balanced game where the CR system actually works or whatever. Thirdly, because I think when player options are banned it is often the influence of the DM rather than the table as a whole, and when a DM, who already has absolute authority over every non-player creature in the game, feels they need to also put a bunch of broad, blanket limitations on how player characters can develop, I do not generally consider that an encouraging sign for how the campaign will go with them.
