The reason to pay is not to keep the domain for sentimental reasons or whatever but because it's an actual security risk to allow someone to jack your domain.
A bad actor who has the domain can very easily setup an identical looking site that can capture login attempts along with ips allowing them to put a username to an ip and and with additional resources could put an ip and therefore a user name to a real name.
They could also use the fake site to do all sorts of phishing scams like fake mutual aid requests, linking to external sites that are also honeypots to get more personal info than most would be willing to divulge on hexbear itself and could also ~~send DMs~~ impersonate any user on the site including to scam users on other servers like lemmygrad who might not know what is going on.
It's easily worth a few hundred to prevent some comrades getting scammed and doxxed.