If GM, BMW, Nissan and perhaps others have high-quality BEVs with at least 200 miles of range by the time the Model 3 comes out (and presumably some sort of DC charging solution), it could indeed be that mainstream Model 3 buyers will not put up with Tesla quirks to drive a Model 3.
But if none of those cars are out, or they are not compelling, or are only available in limited quantities, I think we will be in the same situation we are in today. Even if the Model 3 has issues, if it is the only car in its price class with 200+ miles of range and all the great driving features that go with that type of EV, I think many consumers will put up with it rather than go back to an ICE for the same reasons so few Model S owners go back to an ICE.
True enough regarding mainstream owners.
Take a look at issues most other BEVs are dealing with, and Tesla won't look so bad in terms of minor problems--and I highly doubt those issues will go away with the next-gen EVs of others. The Bolt won't have much pure BEV experience behind it, and the Leaf is still dealing with innumerable issues. Most other cars I follow seem to have a lot of issues too (i3 in particular) with things as simple as charging not working properly.