Are you sure about that sentence? 9.0 had lots of content, including stuff that in other expansions would've come in patches.
Looking past the "expected, normal" features like levelling quests, dungeons, worldquests, there were:
- More rares&treasures, making outdoor SL feel more like a patch zone already in that regard
- 4 long Covenant Campains that each rival Suramar and the War Campaign (I don't presume to claim the individual ones are as long as those earlier storylines, but together they were definitely longer)
- Lots of side content in each Covenant. E.g. I only now really got around to do the Ember Court, and while the scenario itself is a bit boring after a few runs, the accompanying (one-time) quests to unlock more amenities and such (which in turn unlock new events during the scenario) are quite fun, to be honest.
- Torghast is basically something completely new. Yes, tied to player power, but only for a few weeks until you had enough Soul Ash for one or two Legendaries, then it became an optional romp for achievements and cosmetics.
Much of this is grindy and optional, but it
is stuff you could do if you were looking for stuff to do.
Now, obviously you'd have been kinda locked into one Covenant until 9.1.5
on your main if you were progress raiding or pushing keys, but even with that there was enough downtime to swap Covenants for some weeks, and obviously alts would be easy ways to experience other Covenant features.