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.
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 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.
You can compare the effort it takes for history A vs History B. Now, unlike traditional 51% attacks you don’t just need hashing power that instant but instead for very long periods. As such you can compute a minimum history such that actual 51% attacks are significantly cheaper.
That might seem weaker, but Bitcoin’s trust is simply an economic argument. The actual consensus risk is from someone hacking a few nodes thus enabling a 51% attack at near zero cost.