Well things are so complex now that if some provides you source code, it would be so easy to make 0days.

Don’t you think in the long term this would result in better source code and hardened software, similar to what’s happened with the Linux source code?

Right now the NSA just has all the 0days instead.

I agree with you, I am just highlighting companies' thinking why they would wanna protect their source code. security by obscurity. Also having a copy of source could also mean the companies might have cut corners. Many leaks of future products are now guessed from source code. Also it is so easy to copy as well by other companies.

The relative security of open and closed source operating systems implies not much of a difference in that regard.