They've flat out said no to this. There is still enough hubris left for them to think that they can still do it best.
Hubris is right. They are blowing off a major resource no other car company could tap. There are very few Mercedes owners who would be willing to write software for Mercedes without being a BMW employee. Tesla's ownership pool probably has a higher percentage of tech professionals in it that any other car brand.
Nobody can do everything right. Tesla is failing in a few areas. Their replacement parts supply chain is weak, their service center and delivery center networks are straining at the seams with the expansion. And while they are kind of a mixed bag with UI in general, they get failing grades for the infotainment part of the UI (and its internal workings). They are improving on quality, but that is still a work in progress.
As all around cars, their vehicles are the best in the world IMO. Best EV range of anybody, best performance characteristics of anybody except a few extremely expensive supercars, great for hauling people and stuff, great battery management, etc. Because their cars are so great, I stretched by budget from a $30K-$40K car to $100K, but they need to work on their weak areas. Tapping into a resource with their existing owners is one area they could improve quite a bit on the cheap.
I understand the security concerns. There are ways to deal with that. Bring the teams in under NDA if necessary then do a thorough software audit and test before releasing it out into the wild. They need to do that sort of thing with safety critical system software already so they do know how to do it.