Mac has throughout his career struck me as a guy who never really adjusted to adversity. In college he spent 95% of his time sitting back behind a truly dominant line, surveying which of the wide open elite skill players he wanted to throw to. That was never going to happen in the NFL (though SF may be as close as it gets). The only time Mac has looked particularly good in the NFL was a stretch in his rookie year when the O-line was beasting, and they played a stretch of bad defenses mostly. He still had a lot of the same issues underlying though... back foot throws, struggles to drive the ball, late reads, missing open guys, not identifying pressure well.
Last year... lot of good reasons for him to struggle... poor playcaller, line declined (though overall it was more mediocre than terrible), and yes he doesn't have top skill players.
This year...line has been bad, skill players are mediocre.. you have a proven good playcaller though, one who consistently produced good offenses even with bad skill players and he's designing easy spots for Mac.
Now, I agree Mac probably develops better in a better situation, but.... it's not coaching that he never sets his feet. It's not coaching that he loses his reads the minute he thinks he might get hit. That's just something you can't fix with coaching, and you can't tell about a player until you see it. Some guys just don't keep all the things they know they should do when they get (or think they'll get) pressure, some guys do. If Mac was just getting hit a ton, but wasn't imploding, I'd think differently, but he just crumbles, and all his worst habits come to the front.
Yes, Mac is facing a lot of pressure, and yes he doesn't have world beaters at WR. BUT.... some of the pressure is because teams dare him to make the right reads and throws, and they don't think he will. We've seen plenty of Mac bailing from clean pockets due to imagined pressure, plenty of guys schemed open that Mac never even looks at, and plenty of teams saying "Mac Jones can't make that throw, bring an extra guy" or "Mac won't make that throw, safeties stay in the middle".
This is a bad situation for a QB.... it's a worse situation with a bad QB, and I feel like I've seen enough of Mac to say he's a bad QB. Sure drop him in SF on the best talent int he league and he'd put up some nice numbers, but I think it's more likely he's a limiting factor on an offense than a positive one.