The M3 got the trunk because Tesla wanted to give (tall) adults in the back seat a lot of headroom and a feeling of spaciousness (glass roof). I was out this morning in southern California traffic for more than an hour, and I made a point of looking at all the cars to see how many had adults in the back seat — and there were NONE! Not a scientific study, to be sure, but it looks like Tesla obsessed on a feature that will benefit almost no one, to the detriment of all of those people who want the practicality of a lift-back for loading stuff in the back of the car.
It would be really nice if they would make two variants of the M3—the trunk version, and a liftback version with the structural cross beam relocated further forward. It should not be any more of a production hassle than single-motor vs. dual-motor, or standard battery vs. optional larger battery. Both versions could be built on the same robotic production lines. Since Tesla builds each car to a specific customer configuration, there would be no issue of maintaining inventories of cars with different variations (as manufacturers with franchised dealers have to do).
For those who say wait for the Model Y SUV/Crossover if you want a hatch/liftback, I say the people who routinely load lots of tall people in the back seat should wait for those TALLER Model Ys, and give the sleeker, lower M3s an MS-style liftback.
I'll still be buying the Model 3, even if it has only a little hole for a trunk, but it could be a lot better.