The official investigation for both the JT610 and ET302 isn't out yet so we do not know if there is indeed a design flaw with the 737 MAX. We don't even know if the MCAS is indeed the culprit that cause the Lion Air flight to crash. However, if there is indeed a design flaw with the MAX, then I agree that the aircraft should be grounded immediately. So, for now, I will continue to fly on the MAX as I would on an NG even if it the flight that crashed was replaced with an NG.
Well the thing really is, whichever authority or airline decides to keep on flying the MAX, and if nothing happens, they can state sounding really cool that "we knew there was no need to overreact". Really tough guys who make sound and informed, wise decisions right? Everyone in the financial side of things are very happy, pad on the back, well done.
Now, just think, what if there will be another one? Another 150+ people die, because of NOT taking the tough decision? Imagine if you were e.g. working for the FAA, I'm sure you'd be having a meeting regarding if an action is needed or not. You decide NO, we don't need to do anything. The blood of those 150+ people would be directly in your hands. You could have saved every single one of them, but you didn't, and now they are all dead. You could even find out that actually someone very special was on board, maybe even someone from your immediate family was on that plane. And you could have stopped it, but you didn't.
And it doesn't stop there. Lawsuits would follow for years to come, 150+ accounts of manslaughter is not a nice entry on anyone's resume. You could end up in jail, in some other countries in front of a firing squad. Some mentally disturbed family members of the victims could make it the last mission of their lives to have their revenge with you (like the mid-air collision above Germany/Swiss border).
Of course it's rather easy now when all those 300+ dead people are just statistics, in some weird countries were human life is anyway worthless, right? Who cares about some Asian or African victims, they are not even real people, you know. Except, that each one of those most likely did have a family, friends and loved ones, whose lives will be destroyed. There are bound to be some unborn children who will never meet their father. Or some father-to-be who will not become a father after all, instead just a bitter, sad widow.
And like has been pointed out already, we are only talking about a handful of planes, it's not like grounding the entire 737 or 320 families worldwide. So to answer the question in the title; yes, they should consider
grounding the MAX-series. And right or wrong, I wouldn't have the guts to grant permission for that plane to continue flying. Honestly I don't think that we will see another accident, but the risk is there, and it would be more than my appetite can handle. Someone else probably would think differently, and I don't blame them.