I tend to stick with a game when I start it. I don't think I have ever had multiple subscriptions to mmo's at the same time. The shortest length of time I was sub'd to a game was probably SWTOR and that was about 3 months or so.
For me, I look back on the games I have played, and if I had a lifetime sub to any of them I would probably hop back in and play them again. Only once have I gone back to a game after letting my sub expire, and that was Everquest.
I think that offering a lifetime at launch will be helpful to the community in the long run. There will always be people who will be committed to a game and who will play from launch, but there will also be people who will wander off after every new game launch (mmo or otherwise...I've raided with people before who let their sub lapse because a new xbox game came out they were going to play obsessively). Almost every summer, game population goes down.
Some of these gamers come back, some get lured away to greener pastures. I bet more people would come back if they knew their gametime was already paid for.
I also know a lot of people will continue to invest in a game beyond sub (or spend the money they would spend on a monthly sub on cash shop type items if their is no sub). I played a f2p game with my husband a while back, and we definitely spent more money on cash shop items (many of which were purely cosmetic) because there was no monthly sub that we had to pay to play the game.
Having had population problems in every mmo I've played in, every guild at one time or another struggling to snatch players from a small player base in order to fill slots of people who had left the game for reasons NOT including not liking the game....anything that encourages people to continue playing a game is a good thing in my book.