Can they subscribe to Y’s posts? (I mean, without creating another account.)
More generally, though, I agree. Twitter’s aggressive spamming of nearly every page with irrelevant posts (for example, the replies page) is a better reason to switch.
Seems to be unclear, apparently X doesn't do auto-unsubscribes, so you'd still be charged for the subscription. Previously it'd tell you that you can't access subscriber content anymore, but it isn't clear if that's still the case.
More generally, though, I agree. Twitter’s aggressive spamming of nearly every page with irrelevant posts (for example, the replies page) is a better reason to switch.