Bayern can't join ESL if the majority holders (Bayern fans) don't want to because of 50+1 rule.
PSG won't join ESL in the near future as well. PSG is owned by the same owners who own Bein Sports who have the Champions League TV rights until 2024. So there is absolutely no reason financially for PSG to join ESL. It would be impacting their revenue.
Yeah, fans are fickle bunch. Let's see how many years Bayern fans are willing to put up of playing in the 2nd rate European competition, while also walking the one team league known as the Bundesliga. Would wager wouldn't last more than a few years.
PSG owners are all about status, maybe they hold out until the World Cup in 2022, but shortly after that they will aspire to take their project toe to toe against the big boys again. BeIn would already be hemorrhaging money if the ESL was to take off, PSG staying in the competition would not recoup the losses or anywhere near.
Also don't see how it is financially beneficial for either club to play in the equivalent of a glorified Europa League. Would earn much less and would have their players poached by clubs in the ESL.