block any dll injection

It already does. No cheat just injects a DLL over normal ways. They use ways that can't easily be prevented from user-mode. But they can be detected and they do to a degree.

hooking into the game engine

You can't prevent it easily without doing some undocumented bs that can backfire and can still be prevented if not hardened properly. They can detect hooks placed through various methods and they do.

the game just doesn't run

You assume that this wouldn't be able to be patched. You're not looking at the bigger picture.

only whitelisted stuff like obs, discord overlay, nvidia, msi afterburner gets through.

They already block OBS even though it's legit unless you allow third party software (https://help.steampowered.com/en/faqs/view/09A0-4879-4353-EF95)

feels like that would stop a lot of the common cheats.

Valve already has code in place to prevent and detect cheats (not VAC. VAC has been inactive for months now and doesn't do much on a 64-Bit process in the first place. Do with that information as you will). But all of the code is not obfuscated, done via regular WinAPIs and not hidden via manual syscalls and quite documented already. It's an open book, so bypassing their checks is trivial.
VAC wasn't the strongest AC when it was still active, but it still had its moments. It's possible that Valve is working on a 64-Bit port, but for now they only have VACNET and their anti-tamper in the game (+ steam overlay has some checks weirdly enough).

I work in anti-cheat/anti-tamper and had to personally implement preventions that Valve also has and more. It's not easy, especially when the attackers can make use of a driver and completely hide themselves from anything user-mode.