That I'm not sure.
Performance wise Schumacher was in 2012 closer to Rosberg than Rosberg was in any 2014, 2015 or 2016 to Hamilton. Of course if the difference was going to be like in 2010 then it would have been more one-sided. It's too hypothetical, because nobody knows how Schumacher would have adapted to the 2014 rule changes, could have possibly stopped the momentum he started to gain over Rosberg.
I also believe that Rosberg would have won in all three occasions (and I believe all titles would have been decided before the final), but likewise Hamilton would have done the same against Rosberg if not for reliability, so I dont think that it would have been much worse than in the last three years. Dynamics would have been different in any case, given that the background story of Rosberg and Hamilton might have turned things to be a bit awkward in my opinion.
The Schumacher/Hamilton scenario someone mentioned above (which was never going to happen) is a bit more complicated: Performance wise it would have been more one-sided probably. This would have been much more interesting in terms of dynamics, because Schumacher is a character who can play the games Hamilton does.
This is true, but I didnt meant that (though these were exceptions from the rule). They focused the sport too much on Schumacher, F1 was basically Schumacher (and before someone is saying this: No, the British coverage is nowhere near this level regarding Hamilton)
Reading your post (a good one!), I think Schumi staying on and being in the dominant Mercedes would have been a lose-lose situation. Personally, I like the current F1-regulations but both with the anoraks (give us back the screaming V8's!) and with the general public there seems to be a declining interest in F1 and in motorsport in general, and evena kind of... tiredness around the sport. A kind of: okay, what is new?
If Schumi (by some very clever politics) would have beaten Rosberg: the story would be (one of many negative): 'Geez, it is only about the car. Even an old geezer can win it. By the way: when he was young he 'only' won because of the Ferrari-dominance and the FIA-ferrari assistance.
If Schumi (by some miracle) would have beaten Hamilton the story would be (one of many negative): 'Geez... The new generation of F1 drivers is half as good as the Senna-Schumi-Prost-era... look how a past his shelf-live driver beats the supposed best of his generation.
If Rosberg wins: Schumi was never that good. Or Rosberg is lucky to have an old geezer as a teammate in that dominant car.
And so forth, and so forth.
So, no, what we have to put our hope in is drivers like Verstappen and Ricciardo, perhaps Ocon... A fantastic bloody battle between them and Hamilton would be more than excellent. We have to attract a new audience and rekindle the interest in tired existing fans (who complain too much, in my view).