More than 300 dead is largely worth an abundance of caution
This decision is entirely normal. One crash is a subject of commiseration, two of the same kind with the same airframe and on new planes and it is time to realize that something is likely very wrong.
I'm not pouring fire and brimstone on Boeing though, they have one of the most complicated jobs on the planet. I'm sure they'll find out what is wrong, if only because now that the planes are grounded, the pressure on them is skyrocketing.
So yeah, they'll solve the problem. They have to.