Linux is already used everywhere, from servers to satellites to phones to infrastructure. There's already a huge incentive to find exploits, moreso than Windows devices.

I do think more desktop-oriented exploits would be found if more people used Linux desktop, but I think that's more down to distro fragmentation and not every distro maker being as competent as others, or not having the manpower to keep up with development, as opposed to there intrinsically being danger in people seeing source code.

NSA would definitely want to keep some linux exploits around

And they'd be spotted in the source code and patched. If the code is proprietary, you can never trust that there aren't backdoors.