Riker's fatness may be an even lamer excuse to put a show in an alternate reality than a slightly different stardate system
I think you're making an incorrect assumption about TOS: That it's the pure/correct/original timeline to begin with. Even ignoring Enterprise (which conclusively proves TOS/TNG/etc. to be the result of several major shifts in the timeline), TOS itself interfered in the past more than a few times (they stole whales! Left a phaser behind! Invented transparent aluminum - all in just one film) TNG went back to a 19th century full of aliens from the future who were killing the homeless. Sisko replaced Gabe Bell.
Don't forget Captain Braxton's 29th century time police and
Crewman Daniels' 31st century time police who always cause far more trouble than they stop.
Long story short: The timeline is screwed. Utterly. As time goes on, more and more spiecies will learn how to go back, and make it worse. Unendingly.