My point is: whenever such a catastrophic event happens we want to know about it as soon as possible, to try preventing the damage.
That is why having only one software implementation is a very bad idea.
That's why I said in an earlier post that having the
same person run multiple software is good. I don't think that if everyone was running different software that the problem would be noticed significantly faster than if everyone were using the same software.
But you just described Sybil Attack which can happen with or without multiple implementations of bitcoin. It doesn't even need a vulnerability or hashrate to happen.
It's easier to perform this type of attack when you can get other people to voluntarily be your sybil nodes. That's what multiple implementations do in this situation: other people are voluntarily being the sybil nodes.