Sounded to me like he's against unattended player arenas.
I believe what he meant is that he doesn't want Custom Arena just staying up by default, so that people don't needlessly take up resources because they didn't know how, or forgot to shut it down. He just doesn't want every little kid who starts one to play around leaving it up forever because Power Rangers came on TV and he closed the client without hitting shut down arena button.
Charging a nominal fee that you would specifically have to sign up for would keep the Custom Arena working as-is for the vast majority of players. You would have to intentionally sign up ahead of time for the extra surcharge to have the capability to persist a Custom Arena. 99.99% of the players wouldn't choose that added charge. Kids wouldn't accidentally leave them on when they leave. They would automatically shut down as they do now.
Someone who has added the persistent Custom Arena account add-on is specifically choosing to do this. It wouldn't just be by accident. And they would be limited to one persistent arena, or perhaps additional charge per arena they want to persist. It is the best of both worlds. It allows the user to feel a little cost for persisting an arena to prevent it being done frivolously, but allowing it for user who have a true use.
Hence why he was warming to the idea, because it allowed the capability, with a self-enforcing limiter for waste.
$0.02.