Generator monk is indeed the best monk build for higher GRifts.
Unfortunately, after the double nerf to the Static Charge monk, the class went from the best to the worst at GRifts. Blizzard just didn't replace that build with anything close to it.
Case in point, here is the spread on the Season 5 Leaderboard:
Top Crusader is at 91, Top 1000 is 80.
Top Wizard is 89, Top 1000 is 78.
Top Barbarian is 88, Top 1000 is 74.
Top WD is 87, Top 1000 is 75.
Top Demon Hunter is 84, Top 1000 is 74.
Top Monk is 86, Top 1000 is 71.
I would say the popularity of the builds revolves around the best classes for the patch. You see most of our community focusing on Wizard and Crusader builds. This is where all of the time and effort is going when guides are made, settings are posted, etc.
I run a multi-bot setup so I can't give you a ton of sage advice on how far it can be pushed in Solo. Last time I ran it, I did a 68 to bump my blood shard cap up and didn't try again. What I will tell you is that I am slowly giving up on the class for 2.4. It's a support class for me at this point. And I am about to roll a WD or Crusader on that account to replace it.
Monk is still good for speed farming, but top end GRifts are tough for all monks, as evidenced by the disparity at the top level:
Crusader 91/80 compared to Monks 86/71.
I hate to say it because I have loved Monk since before RoS, but it's time to roll another class if high GRifts is your goal.