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Games like WOW, and now SWTOR, have free limited trials where new accounts can play the game up to say a quarter of the level cap for free. This is indeed a great way to get new blood if your game is solid.
Are they missing out on getting unsubscribed players to come back. Some players may come back and level a new character and after hitting the cap may decide to resub. All they'd have to do is prevent players from logging in characters above the trial cap. I guess it would prevent issues where those players could be guilded where new characters couldn't, but there are fixes for that as well.
I never understood why games with free trials didn't open this perk to unsubscribed players in hopes of getting some of them back. In the case of SWTOR, I mainly wanted to see the populations and try to rename my legacy, as I assume it was lost on the transfer. But once I returned I may have decided to sub for a month.