The risk is probably way smaller than intentionally collude to make a shitton of money and let it crash and burn.
The perfect exist scam if you want. If china for example would actually put an ultimatum on chines miners to shut down. It could potentially make perfect financial sense to leave with a big boom and make as much money as possible before closing.
Validator nodes aren’t the weak points. It’s as you say the mining pools themselves, internally they need to be coordinated and have access to the Bitcoin network so they can’t be air gapped. So while all major pools have solid network security as they’re major targets, it’s still an actual risk.