Re: "So someone decided use different battery size for Model 3 "
"The real problem is some dimwit thought that it's a good idea to have different battery packs for different Tesla models, the only compromise there is with common sense."
It's unavoidable when the technology is evolving. I wouldn't want still to be on crossply tyres with drum brakes, non-power steering, pushing 8 inch floppies into a 30kg floppy drive and loading paper tape to get programs to run. But that means breaking backward compatibility.
My own feeling fwiw is that Musk felt the big car companies breathing down his neck and decided he had to compress timescales beyond what was actually achievable, before GM and Nissan got the battery range or PHEVs became mainstream and killed the BEV market. As it looks as if 2019 could be both the year of the mainstream manufacturer EV and the reasonably priced PHEV, the window is very narrow. And one thing Musk doesn't seem to have grasped is that if you take on more people the project takes longer, but if you don't take them on, you can't get the volumes.