I've tried to explain it in my comment above.
Yes, the rate of account compromises is a metric we can define. But attestation doesn't directly or invariably improve this metric. It may do so in some specific scenarios, but it's not universally true (unless proven otherwise, which I highly doubt). In other words, it's not an immediate consequence.
It could help to try to imagine a scenario where limited choice can actually degrade this metric. For example, bugs happen - remember that Infineon vulnerability affecting Yubikeys, or Debian predictable RNG issue, or many more implementation flaws, or various master key leaks. The less diverse the landscape is, the worse the ripples are. And that's just what I can think of right away. (Once again, attestation does not guarantee that implementation is secure, only that it was signed by keys that are supposed to be only in possession of a specific vendor.)
Also, this is not the only metric that may possibly matter. If we think of it, we probably don't want to tunnel vision ourselves into oversimplifying the system, heading into the infamous "lies, damned lies, and statistics" territory. It is dangerous to do so when the true scope is huge - and we're talking about Internet-wide standard so it's mindbogglingly so. All the side effects cannot be neglected, not even in a name of some arbitrarily-selected "greater good".
All this said, please be aware that I'm not saying that lack of attestation is not without possible negative effects. Not at all, I can imagine things working either way in different scenarios. All I'm saying that it's not simple or straightforward, and that careful consideration must be taken. As with everything in our lives, I guess.