Sad story is sad, but PayPal is not going anywhere. When eBay introduced earlier this year that they will replace PayPal with Adyen as their primary payment processor, at that time PayPal's non-eBay volume was already growing faster than its eBay volume. This year PayPal signed a deal with Walt Disney, QVC and Dillard's to be a payment option on their sites. PayPal definitely doesn't strike me as a company, which could go under in the foreseeable future.
Believe it or not, high FnF payment rate can trigger account reviews, this is mentioned on this very forum countless times or if that's not enough, Google can reveal more. I haven't read anything about this lately, but it's like this since a long time and if they put more emphasis on this recently, then there's the explanation.
It can be that you were not selling any prohibited items, but you were still selling, the FnF payment option is clearly not for selling. So who is at fault?
This issue is already non-existent outside the US though, because of the recent fee increase of the FnF payments in non-US countries, so it doesn't worth it anymore to use it to send/receive payment when buying/selling.
You shouldn't have kept thousands of dollars on your PayPal balance either, that's also a big no. Leave them some money, never empty the account, but definitely don't keep thousands there for long.
Lesson learnt the hard way i guess.