The solution I posted isn’t based on consensus, like Bitcoin even 1 node with a stronger history should win.
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.
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.
BTW if you would have full control over any 4 XRPL validator nodes at your choice you could do absolutely nothing. No double spend, not even halt the network, nothing at all. You could turn them off and only a few nerds who constantly check the network would notice. User of the actual network would not.