It is totally up to the agend of the other companies. Theoretically, they should be able to since tesla has open sourced all its patents but I don't think many companies want to do that. EC cars are still a niche that only very few people drive. And it still requires tons of R&D which might not be worth it for the other companies.
What is really going to be interesting is how well other car companies can build EC cars. I personally like the look of the tesla model 3 way better and at a similar price point, it is a no brainier to get a tesla for me (if I had the money). Also, I'm really interested in autonomous driving and tesla seems to be far ahead (I know lots of companies are catching up but autonomous driving is still in the higher end models.