MoP had more. WotLK likely had more as well.
Ie, WotLK added achievements. Show me an equivalent big system in Legion. There's none. Mythic+ is small peanuts compared to achievements, same for artifact.
Or take MoP. It added pet battles, a huge system. What did Legion add apart from infinite difficulty modes everywhere? What new gameplay did they do? None.
But let's even talk raids. Fine, Legion's tier 1 is 20 bosses and MoP's tier 1 is 16 bosses. 20 is more than 16. But it's been 6 months and that tier 1 isn't even fully there yet, it is still unlocking. These 20 bosses didn't even arrive completely yet. And after they arrive, there will be several more months where there are no new bosses coming. Yet if we were in MoP, we'd already have tier 2 opened. MoP wins again.