The usual argument given for wanting programs to halt is that if you can't solve the halting problem for your language you can't prove anything, so we want languages where we can prove things.

But you don't have to have programs halt to solve that. It is a perfectly valid solution to the halting problem that all your event handlers halt and your mainloop doesn't halt (or only halts on receipt of a signal, and never on receipt of a request, or something). And then you have your ability to prove things back.