If you cancel your subscription and stop playing the game for some time, you can come back whenever you wish to start paying and playing again.
They are not going to delete your account, nor will it be more likely to suffer some technical problem than an active account.

If you have a house in the game you will lose it and all the furniture in it if you are away from the game too long. (45 days without entering the house, then 35 days to pick up any furniture.) But that is the only thing you risk losing if you go inactive.

The problem with overpopulation is not the total number of accounts - there must be millions of inactive accounts - but the number of active players logged in at once.