It gets into a "define unlimited" discussion. You could give "unlimited" usage, but still "limit" both concurrency and throttle speed. Even if you run 24/7, they still have knobs they can twist.
The limiting thing now is total tokens, so unlimited would mean infinite tokens.
And it's not hard to spin up to infinite tokens, one agent and one command can orchestrate it.
I'm thinking the persistent agent is something that can't scale to multiple agents. Imagine only one agent you can invoke as many times as you want on a single account, but it cannot invoke subagents or scale itself. It's just always there ready to do work when you need it. But it behaves as "one".
This would be trivial to implement. Of course, if you have the money, you can have multiple $500 subscriptions and have multiple persistent agents.