I wish people would stop saying that.
BTTF is NOT saying that wealth brings happiness. Yes, the McFlys ARE richer (not rich, just richER) in the new tiimeline. But that's irrelevant.
The McFlys are happy because they're stronger and more confident. All of them, not just George. And that added strength and confidence is the reason they're better off.
About that bit in BTTF2 when Old Biff is visibly ill as he gets out of the DeLorean: I like the explanation that the reason he's sick and being erased from existence,
because Lorraine shot him sometime in the past.