Instanced housing doesn't work the same way wards do. It's load on entry, much like dungeons and other instanced duty content. Player wants to enter - instance gets loaded on the instance server. Player leaves - instance gets removed from the instance server. A player can't be in 2 different instances at the same time (short of having multiple service accounts). It's a very efficient use of server resources.
Wards on the other hand are like the open world zones. They are always there, player present or not. It doesn't matter how many of the 60 houses in a ward are owned or how many players are present in that ward - the ward is going to take up the same server resources constantly as long as servers aren't down for maintenance. Sadly, many of the wards are empty of players more often than not. It's pretty rare to see more than two or three players in a ward at a time short of a decent sized FC getting together to meet. Once a player enters a house, they're on an instance server and no longer in the ward. Our house interiors are already instanced.
You're right that technically the servers can't handle unlimited instanced housing but that's not going to be a problem unless every player decides they're going to sit in their housing at the same time. That's extremely unlikely to happen because housing itself doesn't offer much to do in the way of content and not every player is even logged into the game at the same time. If I look at my FC roster, friends list and linkshells, it's rare to see more than 10% of those online to be in housing whether it's the wards, a house or an apartment. The majority are in other locations.
If instanced housing was going to create problems for server resources, we'd see that problem in the MMOs that use instanced housing. It's not happening, not even in ESO where every player can own every single housing location in the game if they want (right now that's over 40 separate housing options).