You should always own the domain your email is sent to. If this is the case then the solution to this problem is trivial with DKIM/SPF, if this is not the case then it does not matter what third-party provider you are using because this can always happen. Your complaint has nothing to do with fastmail.
Other email providers don't allow recycling of account names - once they're gone, they're gone.
That said, I do agree that using a domain you own is better practice. However, I have been burned there before - I used a .eu domain for pretty much all of my email sign-ups for over a decade, then had the domain yanked away because of Brexit. Yes, my fault for not realising that this would happen (I lay some of the blame with my domain provider for not mentioning it to me at all).
Well it can expire if you let it expire. There is nothing that can absolutely guarantee you keep you email address, but owning a domain name (say with a 10 year prepayment) is damned close.
