Being wounded also prevents you from playing—much more so than prison does, in fact. Do you consider wounding to be akin to a forum ban? Do you think we should remove it from the game?
Prison is a part of the game. It has been since the beginning of the game. There is nothing about prison that has changed to make more people leave the game. There have always been a few people who think that being in prison for a week is the worst thing EVAR, and there always will be, but that doesn't mean it's actually a detriment to the game as a whole.
Furthermore, this change—making sure that new players can't be put in prison for longer than a few days—should significantly help in making sure that new players aren't kept from the game, and thus increase their enjoyment.