There is a chart somewhere that shows the % of Masternodes you would need to control to get a % chance at deanonymizing a private send.  It's a bit old, it stops at 4000 masternodes, LOL, which we now have 4200+ but still, it shows how fruitless it is to try, and it would be a waste of money and resources for any large nefarious group to try.

And that is why Masternodes are collateralized.