>The correct move here is to act even if they think the certainty is low.
What is the correct action though? The root cause of the first crash is understood to be a complete lack of training on a single aspect of the flight control system in a particular set of circumstances (one that happens to occur in the most dangerous part of the flight envelope). From what I understand about this issue, the correct action is to institute an immediate crash course (pun intended?) to train pilots on how to recognize and manage the actions of this subsystem unless further, currently unspeculated issues with the sensors and software are discovered.
>(T)he risk of grounding planes is operational inconvenience
I think you're overlooking the risks involved in adding significant pressure to flight schedulers (who have to get planes in place to make up for missed routes), maintenance crews (who have to provide more flight hours for the remaining fleet to make up for the missing planes), and pilots (who will have to fly something different than what they've recently been flying).
Think of how many opportunities for a mistake to happen would arise from changing something like the route, time, and (sometimes) vehicle used for the daily commute by every worker in a 2,000 person company. Tighter timetables, increased flight hours on the remaining fleet, and the need for pilot's to shake the rust off their skillset flying an airframe they haven't been in for a while are all significant risk factors that have played a part in the vast majority of flight accidents in the modern era of air travel.