I'm trying to wrap my head around the fungibility. A perfect peg means the value on the side chain is exactly 1-to-1. But there is *less security* on any side chain, because there is a non-zero chance of drivechain being "undone". So people value side chain BTC *less* than they do main chain BTC.

Of course the purpose of the drivechain is to enable more functionality on side chains. So the decrease in security may come with a benefit of privacy, functionality, or scalability.