[msangi]

Why is this a limitation only of type-checked functions?

The problem is whether the function can or cannot handle the content of the new message and that's completely orthogonal

[swhipple]

The difference is that attempting to add code to a live system that mishandled an input (either from a non-upgraded node or an upgraded one) would result in a type-error and reject the upgrade.

You could still use Erlang's crashing and supervisor technique, but you would have the additional benefit of using static typing across a distributed system (where each node may or may not have received the upgrade yet).