I think a limited-offer lifetime sub would draw in more people and be a compromise to many people who feel annoyed they are paying for less than a complete game (I'm not annoyed myself but I know alot of people are). SE could offer the lifetime sub option for maybe a month or so, and then drop it, allowing them to guarantee monthly income from players who join later. Personally, I think it would be a nice gesture to everyone who has stuck with the game from launch.

My $0.02.