My two cents worth!
Here's the thing Roth didn't have to make the Donship happen, obviously he can't but that was never his intention, of course if that did happen, he would have Don Fredo [albeit Roth's puppet]

All Roth wanted was Michael dead exactly what he set out to do and his job done
He goes back to living his retired investor on a pension exactly as he planned

Fredo was so consumed with being stepped over Roth knew that is the only carrot to get him, Roth's pawn, on board

I reckon Lana and Turnbull nailed it
Originally Posted by Lana
  • Fredo's “there was something in it for me -- on my own"

I doubt the astute, meticulous Roth would master plan such a do or die, critical operation of murdering a Don, in his own home, in his bedroom, without specifics [on vague / implied] without making it clear to Fredo that the reward is Donship or Roth would have instructed Ola to make sure Fredo listened good and inferred it [Ref: Turnbull] Hooked!

They need to ensure, imperative Fredo opened the drapes thus identifying Michael and Kay's bedroom, which is obviously of vital importance to the Tahoe shooting without which there can be no assassination and only the Donship carrot would get the job done