That's the old "slippery slope" argument, and it's a logical fallacy in this case, and has been argued exhaustively elsewhere. This 90-day thing has been the policy for how many years? And yet everyone does NOT do it. So something MUST be stopping them.
And what would that be?
1. it's inconvenient.
2. Once the account expires, the householder can no longer get into the house - and it sounds like pretty soon once the account expires, nobody can get in the house, if we understand the coming changes correctly, so there goes a part-time house for part-time play on other shards.
3. Most players do want to support the game and pay full prices for the accounts they regularly use, and as Atlantic Realtor says, these 90-day accounts are just extra money for BS that we would not pay for if the policy were to change.
So there are at least three things that disprove your thesis that "nothing currently stops EVERYONE in the game from doing the same thing."