As Max Tegmark mentioned on this Rationally Speaking podcast quantum immortality might only work if the universe is infinite.
I don't have bandwidth for a podcast just now; so 'infinite' in what direction? If the number of MWI timelines can be divided infinitely, then that seems like it would suffice, even if the universe is finite in many other ways.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.