Yuengblood
Good question. If you sub, your account is locked into P2P mode and you can't go back to F2P. Some people like freedom and flexibility and don't want to get locked in.
This is something i have never really understood tbh .
I would think that letting people sub here and there , then when it expires locking the skills to whatever the Unlimited Trial cap is would work better .
Wurm Online ( not trying to promote it ) does exactly that . It has an unlimited length , skill locked trial where all skills are capped at a value of 20 ( max being 100 ) . If you subscribe ( buy Premium ) , you can raise your skills above 20 , and also build things like stone houses , ride carts / sailboats etc ( which are all just out of skill reach of free players ) .
Once your Premium expires , you can still log in and play , but your skills become artifically capped at 20 again ( note you don't lose them , they just become blocked ) and you lose the ability to do the things a premium can , until you resubscribe once more .