There's good news here: the Kernel Self Protection Project is a team of Linux devs who have been successfully upstreaming good, compatible ideas from grsecurity.

https://kernsec.org/wiki/index.php/Kernel_Self_Protection_Pr...

https://lwn.net/Articles/698827/ (note the admission in the comments by all sides that KSPP is essentially about upstreaming all the upstreamable ideas in grsecurity, to bypass the sort of yelling that you see in those comments)

The trouble is that the incompatible ideas run contrary to Linux's philosophy of compatibility / "don't break userspace" being the highest virtue, but that at least isn't about personalities, just differing design goals.

You've got it a bit wrong. Brad Spengler (aka spender) and much of the grsecurity team's goal is making Linux more secure by working on grsecurity. They're not interested in totally rewriting large portions of Linux technical debt to make Linux more secure, but upstream demands they do it. Is spender often a bit of a prick, yes, but is he virtually 100% right on security things, also yes. It wasn't really a rejection from kernel mainline so much as they don't have any interest in submitting it. You can't have rejected what you never submitted in the first place.

Fast forward to now and you have kernel heavyweights like Kees Cook (former Canonical kernel/security lead now chromeos security badass extraordinaire) working on bringing these features into mainline via the aforementioned KSP projects. In the end, Linux is getting more secure, and it is largely in part to the hard work of the entire GRSec team, even if they're a bit abrasive to work with.