To be fair, if Ubuntu MATE was to cease existing as an official flavour, nothing stops someone from building an "Ubuntu MATE Remix" spin (like the 14.10 and 14.04 times). 
Not sure if (or where) the script to building those ISOs are.
If that happens, then this community could adapt into something else or have a migration path. It's our (Martin's) server, so we control the fate to this site. The only thing Canonical can control is the brand name (Ubuntu MATE) or Ubuntu trademark in the event it is no longer an official flavour.
At the moment, there is no cost to hosting this site (it's sponsored by Bytemark), but that could change if we were to become purely community based or we'd need to downsize... or check with the leader if the Patreon money can cover the costs.
You see, if it was me leading a new distro 'remix', I would pick Arch Linux as the base. It's really easy to build custom ISOs. I also maintain a gtk3-classic package which brings back older GTK behaviours (which is a pain to package for Ubuntu). Plus, if we were snap adverse, then it wouldn't include them - native packages all the way (Octopi instead of Synaptic)!