There needs to be a reason to log in for 4 months even after you've killed all the bosses once and done all the quests. It's either lockouts, long currency grinds or RNG. I find lockouts to be more player-friendly then the other two.

Of course people don't have to stay subbed for the entirety of a 4 month patch but the lockouts are there to reward those who want to. Players who like to unsub don't have to get any weekly locked gear because in the next major patch they can just get the next uncapped loot before unsubbing again.