The 70/80D was always a great camera for people stepping up from a Rebel/Kiss. The price wasn't crushing, you could use your existing lenses, has lots of features and it was a major step up from the Rebels. I think this will do well for people still looking for that in a DSLR. Having said that I am not sure there will be a lot of demand for the 90D from people that allready have a 80D/7D. Many are looking at mirrorless or won't see this as a large enough upgrade.
Because of that I don't see this as being a preorder monster. But I think it will be in the line up for a long time and I think they will move a pretty good number of units over time, though not as many as the 80D. DSLRs just make more sense in some situations like when battery life is a factor or in action situations. Crop makes sense from a size, weight, reach, cost perspective. If my 80D fails I will pick one up even though I prefer mirrorless. I want to love the M series but I just don't. I am happy for people who do however.