Considering that they're still iterating on and launching satellites, it doesn't really matter that they'll only last 5 years. It isn't like they're launching the constellation and then sitting on it waiting for the satellites to fall out of the sky.
For now they're having to launch a Falcon 9 twice a week to build up the constellation, but with Starship intended to be able to launch the equivalent of several F9 launches in one go, maintaining the constellation will become much easier.
