Unless you're arguing that old BG fans want only the mechanics and game features that were in BG 1&2 and no more, which is such an incredible strawman that there's not much more to say.
Whilst I take your broader point, I have seen old BG fans arguing exactly that. Had an entire Youtube comment argument with someone claiming that a Baldur's Gate game shouldn't call itself Baldurs Gate unless it's RTWP...
There are of course extremists, but I think (I hope) that these are in the, possibly loud, minority. As a BG and crpg fan, it's obvious to me that it is not one specific mechanic that defines a game. BG3 can be a faithful sequel to the BG franchise without perfectly recreating its mechanics.
In fact, it is impossible for BG3 to perfectly recreate all of BG1&2 because there is no way that WotC will allow Larian to use older versions of D&D. Changes are required, and even can be good, while still retaining the spirit of BG, whatever that is.
I've not seen a single person argue anywhere that Solasta isn't a much better, truer adaptation of the DnD ruleset to a cRPG format than BG3. It even has higher ratings from its players than BG3, probably as a direct result.
But BG3 has sold more copies, more people who bought it played it, they played it for longer, and are still playing it. Of couse that's an over-simplified perspective, but it's compelling all the same...
A more useful metric would be comparing the ratios of (# of copies sold)/($ spent on development and advertising). Of course BG3 is going to sell better; it's the game from the company that had a huge hit with DOSII, it has a much larger development team, and has had a much higher budget. Whereas Solasta is the first game from a development team of ~12 people.
Also, BG3 selling better than Solasta doesn't mean that all of Larian's decisions are justified. If Larian had made BG3 more faithful to 5e, it could be selling 50% better...or it could be selling 50% worse. But I argue it'd probably be selling better, given the near ~universal praise for Solasta's mechanics, and the decent chunk of negativity for BG3's mechanics