The truth is likely a combination of factors. But I believe Elon when he says the range was problematic. Because when does range tend to matter? That's right, when you're on the Interstate going 75 mph. And that's precisely where the Model Y with a smaller battery would start to show it's limitations. But I agree with your other points.
Just realised a possible implication of there not being a short range Y any more...
Is it that maybe orders for the high end version of the Y are so insane that they know there is no need for them to even consider building a cheaper version for a while?
Even a <250 mile range tesla would sell very well, but maybe they literally cannot build the long range high-margin versions fast enough to satsisfy predicted demand.
Nice problem to have.
My opinion is an SR Y would hurt Model 3 SR sales. So until they can build that many SR Y, they would be taking money off the table. After China and Berlin Y factory’s are running, a MR Y will appear at about $44k. The 3 will be further improved or upgraded to MR at the same price as SR.
Also competition is ****ed.
Lots of possibilities, and I agree with @StealthP3D that the truth is almost certainly a combo. But another piece that I haven't seen mentioned is that no one's really towing with a Model 3. A lot of people will tow with the Y. We know towing eats range like I eat delicious pizza, so Tesla may not want to release a vehicle meant for towing that will have a range well < 200 miles when towing even a relatively light trailer. Maybe I'm stretching, but starting at a rated 316 mi will give a much better user experience than starting at 250 when towing.