If this happened then the network is insecure at all levels and we go home saying that was a good idea.
This is the essence of the work being done and that is to make this collusion very hard and unlikely to ever occur. There is a number of factors that work against this collusion and does take time to absorb the details. Search on “primer” for a document that has a lot of good info in it
How do you steal something from a previous owner? Being previous they no longer own it anyhow.
Not sure how. If groups collude they can steal and all that happens is that you can see a history of it. They still stole and obviously cash out ASAP so by the time you see it its too late. The issue is collusion and not ledger in my view.
Already the process of moving coins requires 2 groups of nodes to confirm the transaction.