Except for the fact that they're two different things and there's no valid reason for not letting people keep one but not the other.
To many people, their Instagram account has become a big (and toxic, but that's besides the point) part of their sense of self. To delete their Instagram would be to delete a significant part of their identity. Having to do that to get rid of Threads is emotional blackmail of the worst kind.
If you want to delete something that is part of your Google account, then yes. Like your Gmail address. But not if you want to delete your account in an entirely different service, that only uses Google for the login.
I guess the problem is, that Threads is perceived (and maybe also marketed) as an independent service while it's apparently just part of your Instagram account.
Might depend on who you talk to. Everyone I spoke to about it was under the same understanding that it's integrated deeply with your Instagram account.
I spun up my old burner Instagram account to check the platform out. They tell you that you can only change things like your name from Instagram, the app let's you import all other profile info from Instagram, and when you login for the first time it shows you the thread profiles of everyone you follow on Instagram and gives you a button to add them all.
It's not just an Auth method, it's the same account with the ability to change a few things between apps. Plenty of reasons to hate Threads, but this ain't it.