And here's the thing, there ARE still 'Expansion defining' aspects for their content. Heavensward introduced Flying and established the system to unlock it that is used today. Even if you don't count it, Eureka is still a massive thing that was new to the game that didn't exist before that got added in Stormblood. And we got an arguably better version in Shadowbringers with Bozja, so an update to something that was established an expansion before.
Shadowbringers also introduced the Bi-colored Gems/Fate system, giving more actual reasons to do fates in the world and carried on in Endwalker, and the Trust system, which turned out to be so popular and useful to players that in effort to make the story more accessible to players they're going back and adding it to all the old story dungeons in the game.
And that's not even mentioning the upcoming Island Sanctuary OR the Criterion dungeons that are upcoming and we're going to be learning about this next live letter. Oh, and let's not forget the Adventurer Plate which, while a relatively simple thing, has probably sparked more creativity and fun I've seen in a long time that isn't related to player housing.
Now, does all this have the same impact as 'HEY, EVERYONE, HERE's THIS COMPLETELY NEW SYSTEM THAT WE DESIGNED SPECIFICLY FOR THIS EXPANSION'? No. But I frankly think smaller features, things that are constantly used, updated, or new versions of the same idea (In the case of Eureka/Bozja) are flat out the better option and have more long lasting appeal.