Using @fastmail.com makes no sense to me. If being locked into somebody else's domain is acceptable, then Gmail is a better deal.
And if you still insist on using somebody else's domain, then all bets are off after you stop using that account. First of all because the service provider can always change their policies. Just like how when Yahoo announced in 2013 out of the blue that they'll start recycling usernames.
Fastmail is a paid service, you own that username for as long as you keep paying. The notion that you can take up resources, for free, for all eternity, is only sustainable for big companies like Google. And given how crowded @gmail.com is, I wouldn't put it past them to start recycling in a couple of years.