I'm only an aviation enthusiast and by no means an expert, and fully admit that I could be wrong, and even with my natural inclination to reserve judgement, to me it seems obvious that grounding is warranted purely from a statistic viewpoint.
Compare the release of the 737 MAX 8 with the release of other recent-ish models like the 777 and 787. It was 14 years before the 777 suffered even a single hull loss, and 20 years before a crash of the magnitude of the 737 MAX 8 (loss of all souls). The 787 is almost a decade old and has zero hull losses or fatalities.
The 737 MAX 8 is less than a few years old and already has two catastrophic crashes? I know that it's hard to draw too much information from just two data points, but that's pretty compelling to me.