Well it's Suzuki or Ducati. But it's hard to believe Suzuki wouldn't pay him less than Yamaha, and give him lesser machinery. So ... it's Ducati.
Only ray of hope there is the redesigned engine that is supposed to be ready for next year. Supposedly throws Ducati's engine-as-frame design history out the window, which could make a big difference.
After all, it is not as though the present Ducati is short on horsepower. And there is no obvious reason why a redesigned engine should be slower.
The lure then for Lorenzo would be:
a) a pay rise;
b) undisputed team leadership;
c) the faint possibility that the bike with a year of development and control electronics could be better than the 2016 M1 with control electronics; and therefore
d) the faint possibility that he could cement his legend by doing what Rossi couldn't and winning a title or two for Ducati.
It is all a pretty big stretch. But it is all about the alternatives, and the alternative of spending the next two and a half years on the M1 chasing Marquez, while battling with Rossi for development input and forgoing a pay rise, might not look all that enticing either in Lorenzo's present frame of mind.