That is the attack vector you are worried about?  Really?  Couldn't miners just make the coinbase address a non-existent address and destroy coins just as easily or send them to a non-existent address?

As for why not change it?  It is a hard fork.  Hard forks are never trivial.  In practical terms hard forks will probably never be implemented for anything other than critical (as in "oh my god Bitcoin is going to die") fixes.