Uldir (from Horde POV) and Nighthold are best builded up raids in this game. I'm also fan of Legion patch schedule, I'm not even sure they dropped if on purpose or it was forced by both 8.1 and 9.1 delays.
But split works on many levels:
1) We can have longer campaign that ends with raid, weekly campaign is great tool for telling patch story, but not if it ends on whimper like 9.1 or 9.0. 7.1 did it, 7.2 wasted that potential, 7.3/8.1 had something in between, cause it end with raid but campaign was too short.
2) Every patch bring something substantial
3) That would be something not present in Legion, but new dungeon could be introduced as M0 in big patch, people would get familiar with it, then it would start as M+ on new season 3 months later
4) Catch up gear from new zone makes more sense mid-season than handing everything right away (although 9.1 is doing perfect job using Renown for that).