I see the Honda decision to leave as driven by several factors:
1. The promised simplification of the power unit rules (removal of the MGU-H and other changes) was abandoned after other PU suppliers objected. Honda cited the ongoing cost as an issue with their participation. The cost base of the PUs is still too high, even though they are amazingly impressive engineering solutions.
2. The Covid Freeze severely impacted the Honda delivery path this season, because it seems that they have several performance improvements that they could have implemented this season, but they are not able to introduce them until 2021
3. The current F1 PU rules are not futuristic enough for Honda to be able to justify participation on an R&D basis. Honda's statement makes it clear that they want to funnel a lot more money to EV and fuel cell research in the next 10 years, and the PU rules in F1 are still too focussed on ICE-derived power and energy reclamation.
Once again, we see the "in out in out" nature of major manufacturer participation in F1. This the fourth time Honda has entered and exited F1.
I doubt that Honda will be interested in selling the IP of their power unit to another supplier. They always like to keep R&D in-house. The only time they passed IP to another entity, it was to Mugen.
As for who gets to supply Red Bull and Alpha Tauri - a "cat, meet pigeons" scenario. The rules could oblige Renault to supply both Red Bull and AT again after 2021, if no other manufacturer can be persuaded to supply the teams. I cannot see Red Bull and Renault working together after their previous toxic working relationship. AT and Renault, maybe.
As has been pointed out, Renault might elect to withdraw rather than work with Red Bull again. Somewhere along the line, that bridge was burned comprehensively. Who is most to blame is another question, but when two parties fall out, usually there is blame on both sides.
Ultimately, the exit of Honda is an indictment of the F1 direction taken with the PU rules from 2014 which were focussed on tempting major auto manufacturers, who wrote the rules for themselves (such as insisting on a V6 ICE when the original proposal was for a 4 cylinder ICE), and the resulting R&D costs meant that no specialist independent supplier could afford to play (PURE tried, but could not get funding). So now F1 is down to 3 suppliers.
Major series can run with 2 suppliers (see Super Formula in Japan, which has Honda and Toyota), but the rules are written to define much simpler PU configurations which keep engine supplier costs down (same as in Indycar, whose PU is very basic compared to the current F1 PUs).