I've often heard the criticism of the ending being too overtly materialistic, and honestly I don't really agree with it. Indeed I think it ironically betrays a rather shallow, superficial way of thinking.
Marty's family isn't happy because "now they have a BMW". They're happy because 30 years ago, George learned his own self worth and stood up for himself. He gained the confidence to put himself out there and pursue his creative passions. That made him a more emotionally present, and supportive husband and father, as well as a more positive role model for his sons. Lorraine consequently didn't get dragged down by 30 years of watching the person she loved get kicked around and do nothing to stop it, leaving her to fixate on a toxic sense of nostalgia (and booze) to keep going. Marty's siblings consequently didn't turn out to be so cynical and self-involved, because they had supportive parents and learned to look out for each other too.
Yes, the house is nicer, they're financially more well off, but those things came from what made them happy, not the other way around. It's still the same house after all; it's not like they're suddenly living in a mansion. They're just living more comfortably within their means (and the BMW is probably just a company car.) It should also be noted that George isn't some jet-setting successful author; we literally see his *first* novel get delivered. It's taken him 30 years of hard work to get to where he is, and he's just now getting to the point of realising his dream.
Indeed one of the first things we learn about George at the start of the movie is that he's a hard worker and *good* at his job. How do we know this? Because we learn that not only is he doing his job, but Biff's job for him too, likely with Biff taking credit for all of George's work, thus keeping him in a subordinate position. This George never learned to stand up for himself and so he toiled away under the thumb of his highschool bully. Once free of Biff, it's easy to see why he's able to surpass him in every way, and more to the point, why Biff isn't working at the same company, but working as an independent trader; he can't actually do the job in the original timeline without George doing it for him.
So while it's easy to look at that final scene and declare "it's entirely materialistic!", it's perhaps not a very insightful takeaway. Yes they have nicer stuff, but that's just a visual shorthand, nothing more.
As for Stoltz's assessment of the finale; he's honestly not wrong, and if this movie had a more down-to-earth, gritty tone, then that's absolutely the way to go. But it's not trying to be that kind of movie; it's something of a fairytale. The resolution to the final conflict is literally love's first kiss! I'm not sure how they could have been more explicit as to the tone and themes here.
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