The answer to "Could someone else make this thing I just made" is always "yes", eventually. We have patents to slow the arrival of the "yes" answer enough so that the first person to do so gets to make a bit of money.
But in this case (and most other cases) there's more than one way to do it and a lot of relevant technology, a lot of which is general car technology. And in every case, sooner or later, the huge company with a huge patent portfolio and huge expertise in manufacturing is going to win the "lowest price point" game... if they want to.
At the moment, the big players don't think there's a big enough market to make it worth their while to compete aggressively. At some point that will change, and at that point GM and other huge companies will develop, licence or acquire whatever technology they need. At the moment, Tesla is selling a niche product. That's great, but it hardly the same as producing electric cars for everyone.
Or, to put it the other way round, does anyone see Tesla scaling production up to anything like GM's level while GM quietly hands them market share and eventually gets out of the car business?