I can no longer expand my city.
Your Ch15 city and my Ch16 city have the exact same number of expansions placed. I had plenty of room for Ch15. Regularly climbing the Spire for 2 years also got me premium houses and workshops which saves loads of space, too. I have now stored all my premium houses and some premium workshops and function with good pop/ok culture event bldgs and 2 premium workshops; that also saves loads of space.
I, too, bought premium expansions with diamonds earned from the Spire, but I did it in lieu of placing the research/province expansions (because of the lower hit you take from increased costs in Spire/Tourney now). I have 1 research and 6 province expansions available now for whenever I get ready to proceed in Ch16. I also have been conservative in how many provinces I complete since early in the game. I'm at 463 completed now; 440 were necessary to open Ch16. I scout provinces constantly, but only complete them during events for quests, so that has kept my 'completed' number lower as well.
There's a limited amount of space for all cities; the available grid is it for now and will likely require a great deal of work to expand farther. The available space is split between research, province and premium expansions. There are 55 research expansions, 65 province expansions, and 43 premium expansions listed in the Wiki as currently available through the last chapter released in the game. The remaining expansions will be added when Ch 20 is released and allocated to all three of those categories of expansions.
I don't see how the company could make the decision to change some of the premium expansions to province expansions. There are players who have purchased all 43 premium expansions and wait for the next Chapter release to purchase the next one(s). They'd be rightly upset if everyone else could now get them for scouting more map provinces...