As it is open source this just takes some time as well blood, sweat and tears.
There are several long standing and well maintained open source forum engines out there, all of them have there own strengths and drawbacks. Some are more modern, others do feel more traditional.
The issue with migrating to another platform is that a) the new software will off course behave differently, so there will be a learning curve and b) some stuff maybe will not get over. Like custom reactions, reaction points or whatever. So getting most over is normally quite easy, but getting most stuff over people do care about takes some programming, rinse, wash, repeat. We've got about 293K of posts and 1.9K users. Technically the migration to a new engine is done quickly enough, but the finishing is what really takes time and attention then.
There are forum engines around, which ship with official migration scripts from Xenforo to them. Since this script comes from the dev then it is quite well honed out and does a lot of stuff.
Discourse, which I setup for a while and you might remember, is one of them. Discourse is everywhere and totally opensource. It comes for example with light and dark theme built in free of charge, while the dark theme here is $60. The probelm with Discourse is not if it has a function, but more likely where to find it.
Then there is
NodeBB, which is a little bit more traditional. Other stuff is e.g.
Flarum, where community scripts are around but too much core functionality is a plugin only. Or of course the granddaddy of them all, phpBB.
phpBB has no such thing from , so there you've got to find scripts on Github and figure out what to do by yourself. Why? Because people normally go the other way around: phpBB to Xenforo, almost nobody is going the other way. phpBB also feels really dated compared to XenForo. Also the admin panel in phpBB is a nightmare for a forum as big as this one. It was never meant to deal with this number of users and posts well.
Our consensus is to make
a) first the move then
b) figure out how to ensure a secure run in the future.
For the users of course keeping Xenforo is the easiest and most convenient option. So we are first looking at this. This of course means not only looking at the payments this year, but take reoccuring payments into mind as well. Also to consider the possibilty that funding might deteriorate over time as well.
But I do agree that rebasing this all on a modern open source engine would really take a lot of pressure off all shoulders, and make stuff much easier in the long run.
If I would be some kind of benevolent dictator I would really make the open source move and ditch Xenforo completely, because it makes so much stuff so much easier after that and for the future.
There's also a big thread in the official forums of Xenforo, where a lot of disgruntled licensees do complain about lack of everything from the company behind: lack of momentum, communication, updates etc. There are threads like that all over the web.
Just take a look for yourself:
https://xenforo.com/community/threads/if-xenforos-finished-whats-next.234257/
But it's not about me, it's about what the community wants.