Your misunderstanding seems to be that you think that we think that playing Ellis would "save" the team. "Saving the team" is not the point. The point is to play him enough to properly assess whether or not Ellis can be a part of a Kings team that would hopefully be good enough to compete three years from now. Not to see if he can be a franchise cornerstone player, just a rotational player, possibly a starter, on a playoff team. As of now, that information is not known, because he doesn't get to play.
LaVine is a known quantity. And what is known is that he's not good enough to contribute to a winning team, unless you a) have Prime!Jokić or Prime!Antetokounmpo, b) construct the entire rest of your roster to compensate for his weaknesses, and/or c) build your offensive gameplan around him. The Kings don't have option (a) and, frankly, trying to implement options (b) or (c) would be dumb.
This is reductive and disingenuous. Reducing the argument to "Ellis should start over LaVine" misses the point, which is that LaVine shouldn't be on the team in the first place, and that his (and DeRozan's and Monk's and Schröder's) presence is impeding the Kings' ability to develop the assets that might still be here in three years.