Sitting outside unused is hard on any equipment. It certainly can be properly maintained to mitigate damage but that costs money, and who is paying? If you think of it from a maintenance yard's point of view, they not only have to maintain the MAXes in dead storage, but they have to maintain the replacement fleet as well. When resources get tight, who gets priority? I'm sure that keeping the flying planes producing revenue takes precedence over tending to the sick flock of MAXes. There must be some point at which you are going to have to do a teardown to return the plane into service anyway; tires, lubricants, and sealants degrade. (How square do you imagine those tires already are?) Like diesel, Jet-A is a perfect breeding ground for microbes and partially empty fuel tanks condense water. I imagine they are going to have to pump out the fuel and replace the filters if this goes on for months. I read (I do not know plane maintanence) that it is recommended to drain the sumps every few days on an unused jet aircraft stored outside, this must get spendy when you are doing an entire fleet that is scattered hither and yon.
When the lawyers start playing, the money for this maintenance could dry up pretty fast. Obviously it is not in an airline's interest to pay to maintain planes that they will be returning to Boeing and if Boeing looks like they are going under (no new orders for two months) the contracting yard begins to wonder who is going to be paying for all of those fuel disposal fees. Common sense does not rule in cases like this, I have seen multimillion dollar yachts rot into worthlessness because nobody could agree who would pay to do a little bit of keep alive maintenance . (To be fair, shipyards go bankrupt a heck of a lot more frequently than airline manufacturers.)