To answer your first question on the UTXO set. XBT adopted the UTXO set from BTC and therefore the UTXO's are still important to validate all of the pre-fork XBT and their rightful owners.
On XMR. I believe there is an underappreciated difference between using cryptography to ensure security vs using cryptography to manage supply schedules.
A hard cap does not disincentivize spending. We have infinite demand for resources, therefore at some point we will be forced into spending vs. hodling. If I have a million sats and no water to drink, guess what I'm trading for fresh water? No theory has ever defined this because a truly scarce asset has never existed.
