I'm sure you have read that Red Bull has said they fully understand why Honda has withdrawn. If this was poker, I'd call that a "tell" as to what hand they are holding and it looks like they may be ready to fold their hand. That's speculation, but something to watch.
Super Aguri can be written off as well.
Yes, Ferrari could start supplying engines, but then we'd have to rename if to F-1 Spec Series Racing. Then we'd have to hear all the same complaints we heard through the recent years of IRL and CART. I don't think that's what they want.
I'm trying to figure how Audi's withdrawal from LMS is going to impact F1 and possibly IRL. I think in the least, we shouldn't expect any Audi related companies rolling in to other series. If they hit any one series, I think IRL would be the best match for the points I have made previously.