I cannot recommend fastmail to anyone for the simple reason that if your account expires and gets deleted, anyone can create an account with the same email address and take over your identity. This seems like a massive security flaw.
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.
But on the flip side, good luck making any kind of sensible Gmail account for example, which means either fastmail needs multiple domains or their service will see new customers reducing over the years as people can't get sensible email addresses.
The same thing is true of phone numbers, but it's more obvious that there's a finite number of... numbers... So they need to be reused eventually.
