Have you actually toured every single world to know that there are no plots available?
Right now, Coeurl alone could handle every single player who has posted "I can't get a house" in these forums in the last 6 months. Coeurl is not the only world with plenty of available housing. There are other worlds may not have plenty of housing but they usually do have at least a couple of dozen available for new buyers. It's only a handful of high pop worlds that have no housing available so people scramble when the rare vacancy occurs.
You did touch on a good point about how expansion launches send demand skyrocketing. The problem is that's a short term demand generated by players who aren't going to stick around for the majority of the expansion. Adding more wards to meet their short term demand leaves mostly empty wards a few months later after they sate their curiosity and quit.
Better for SE to expand the apartment system since apartments are loaded on entry instead of being a constant drain on server resources the way the wards are. Apartments aren't popular because people want something that feels like a real life home and not just a rental room. Give people size options so they've got areas to decorate that range between what small and medium houses have (not to mention apartments being cheaper than houses), and apartments would become a lot more popular. It's a better housing solution for the transient players who only show up for the first 5-6 months at the start of an expansion then quit until the next expansion is released or who otherwise take breaks from the game for 2-3 months at a time.
That in turn would leave more houses available for those who maintain their subs constantly and want the houses for the things that only come with a house like intercross gardening or who do hold parties and entertain in their yards.