Yeah they didn't plan on going the expansion route in the beginning, and more for living story updates when bigger content patches every quarter. But I guess that wasn't working out for them in terms of gem sales and living stories basically taking up all their development time, so they switched to doing expansions.
The recent living story release was the first one in a year and a half, the gap from living story 2 to the expansion was 10 months, then another 9 months from expansion to chapter 1 of living story 3, which wasn't that long. Now they're planning on doing chapters every 2 months or so, while a chunk of their staff continues to work on the next expansion, not that they have even finished the first expansion since they've postponed all remaining legendaries to whenever, and moved that team into the living story team.
Right now their priority is to get another expansion out, all this dead time is costing them though, they just had their lowest earning quarter and many parts of the game are still suffering with many new problems popped up with the expansion. The way combat has evolved has affected spvp and wvw badly, particularly wvw. All the conditions and boon sharing and meta and op builds, just ruining the great combat experience they once had with the game.