What a crappy atmosphere to what should be a polite and significant discussion. Joe is a good player but Joe is not a great player. Joe is a great leader-organizer, but Joe can get road-graded occasionally by an OLineman. So... everyone can have a "point" here.
None of us knows why Joe seems fixed at MIKE. These are my opinions, though they have nothing but "feelings" behind them:
1). Linebacking, maybe more than any defensive thing, has a huge "instincts" component. Athleticism is great, but it must be combined with instincts or it's just wild random flailing.
2). In a highly designed modern defense, micro-management of exact techniques and "fits" are vital. If one doesn't do the correct eleven man fit, the defense opens up like waters parting in the RedSea.
3). Joe gets people in correct fits. People might want to doubt this, but I see it all the time pre-snap --- often the guy is Jaylon by the way, but most often a DLineman, even someone like Rochell. Kelly has said somewhere that only Schmitt and Grace do this task like BVG-on-the-field.
4). What about getting bulldozed though? Yes, that happens. It does not happen every play though like some would like to think. Joe is in the correct position a lot, and when so he often plugs a gap and the runner must slide to another slot. Against Navy in the first half, the defense at that point fell to James, who wasn't big enough to effectively fill --- Kelly said this, so we had to go with Martini "who trusted that his teammates would all do their parts of the fits" --- for those who have ears hear --- and the Navy dive essentially was stopped thereafter.
5). Nyles Morgan could slobber knock his gap if he found himself in the true correct one. Often his instincts betray him and he's elsewhere, battling like a wild man in the wrong place. This is no one's "fault". Nyles doesn't have the [unteachable] instincts like, say, Manti did. Few do. Nyles MIGHT get there, but he is not there. He also doesn't get the entire picture as fast as does Joe, and so is probably more confused about pre-snap shifts than the guys he's supposed to be re-positioning. Athleticism is important and studliness, but being in the correct fit is more so. BVG is apparently seeing that we are in the correct fits and stopping plays at a higher percentage with Joe in there than Nyles.
6). Neither Coney nor Martini are thought of as MIKEs by the staff, apparently, and do not seem to have trained [at all] there. My only question, watching Joe get his occasional truckings and seemingly too slow across the field dives, is why haven't the staff tried Grace --- the other acknowledged game manager and good instincts guy --- there? That's the only question I'd ask Kelly/BVG about this.