Forget about new manufacturers coming in, F1 have got their hands full holding on to the remaining ones they've got. Mass OEMs are going to disassociate themselves from F1 as long as the powertrain is internal combustion-based. Renault are making a partial withdrawal by rebranding to Alpine next year, and much of the marketing spend - and therefore the R&D - will be redirected to more future-oriented activity.
Mercedes are already doing Formula E and I can see them selling the team to Wolff when the time is right. There are still independent engine manufacturers,
cider_and_toast - the Chevy Indycar engine is built by Ilmor, Cosworth are still going and Gibson are supplying the LMP2 WEC cars. Many others I'm forgetting too I'm sure. But nobody in F1 sees that as the future.
Without the manufacturers, many of the top drivers (and designers, engineers) would be facing massive salary cuts. But there's no other series - yet - that would pay them nearly as well, so maybe it wouldn't matter? Not to the youngsters, but if you've made your money and won championships, it might be beneath your dignity to drive for 25% of your "worth", as you see it. Interesting times.