It's simple. If you want a chain that gets perpetually more expensive to use and thus more useless as a medium of exchange enjoy your hard cap. If you want a chain that stays nearly free to transact for now and the future you need a tail emission. Monero's has lower inflation than even gold, with perfectly programmed and predictable inflation unlike gold.

They want to keep Bitcoin as a useless tech stock so they convinced the masses that they need to die on the molehill of the 21m cap. Tail emission has been proposed multiple times in Bitcoin's development history and 21m is totally arbitrary. It's a parody that people make that a religious thing "only 21m!!"