787's were grounded in 2013 for battery related issues, when it was a new airplane make/model. There were emergency landings, but no crashes. Here we have crashes, and no grounding. At the time of the grounding there were 50 affected aircraft, lasting from January to April. Grounding 737 MAX 8's would be ~350 groundings, and ostensibly could include MAX 9's.
Looks like it only took two events for the FAA to ground them. I think they were instantly grounded because everyone knew the dangers of lithium-ion batteries catching fire.
