Eh not impossible... just very improbable (in a given world) and certain across all worlds.

"Very improbable" is the typical assumption with MWI, but I think that it is mistaken in most cases dealing with complex systems.

Each wave-function sets limits on what can occur. Wave-functions don't have infinite extents, there are areas with zero amplitude. Each additional wave-function that must meet specific requirements further restricts the possible outcomes. In general, the likelihood of failing to meet the simultaneous condition grows exponentially as the system size grows linearly.

Since quantum survival (avoiding death in some worlds, in some meaningful context) will usually require a very large number of quantum level alternatives to be simultaneously selected for, quantum survival will almost always be impossible.

A person who experiences quantum survival once is very lucky, but almost certainly won't survive the next time. A person who fails to experience quantum survival never gets another chance.

So my conclusion is that quantum immortality is impossible, not just very improbable.