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.
No, it is not an attack vector at the moment, but it makes the rules look more arbitrary, harder to explain, harder to understand and harder to verify. I am writing a program and the total UTXO is not equal to the total "expected Bitcoin" at any time, so I have spent hours to find it out. If it were all in some "blackhole" addresses, indeed, that makes thing simple. In case of not having a full blockchain but only UTXO, I am not sure whether it allow some kind of attack.
I am asking why it is not implemented in the first place. The first such transaction is at least two years after the genesis, so I think there may be some intention to do so.