nip05 are too easily available to be a deterrent to spammers. reply guy would change his npub every few hours or so, getting a nip05 takes a few seconds at most. this wouldn't deter him. Only works because very few people are using this, so reply guy is not concerned about it.
Discussion
You're wrong.
great argument! Could you elaborate on it?
no need to buy a bunch of domains, use the most popular clients that provide nip05 and get one in a few clicks. possibly automatable, if there's no captcha.
ok then. But still seems to me as a lose-lose game. It would probably just alienate your users from the rest of the nostr community, since most would be blocked. Unless you are able to create a great federation of servers/relays committed to the same goals, as they did in mastodon.