I think paid verification is a good firewall and threshold. It is not easy to create a NIP05 spam account with paid verification. The payment may not require 50,000 sats, maybe only 500 sats or 50 sats. A new user can easily get the amount. And NIP05 filtering can be used in normal times. It is only used when facing large-scale spam attacks. In this way, our Nostr notification and reply list can maintain a good atmosphere of communication and exchange. Free verification has no value, and spam will still be rampant. 

Just like now we can not use NIP05 filtering. Because the spammers have stopped attacking Nostr, Nostr's clean communication atmosphere is back.
https://image.nostr.build/f2ec99947b342dbb4df6e5891380c16390382b98a4c79b8ccd67b8b8c029a986.jpg