> It has everything to do with things being closed source. Try doing a Linux kernel major version upgrade with binary-only drivers for key components sometime. It sounds like the only reason GrapheneOS works is because they're 'drafting' off of the kernel and driver work done by Google, not that they've cracked that particular nut themselves. Nothing wrong with that, but it does limit the useful life of a device to the first major security issue they can't fix due to a lack of source code.
As stated earlier, there are no closed source kernel drivers for AOSP, GrapheneOS or even most mainstream Android devices running the stock OS.
The reason for using an LTS kernel branch with 6 years of support from kernel.org is stability. Porting forward the drivers to each new kernel release is entirely possible and isn't a lot of work when it's done incrementally. Not that many changes are even required. The issue is that there are substantial regressions in each Linux kernel release and it takes at least 4 months or more to get a production quality kernel for a specific hardware target with nothing break, the massive CTS/ITS/VTS passing, etc. Pixel 6 uses the Linux 5.10 kernel branch which was the latest at the time, and those LTS branches have 6 years of support from kernel.org and expanded support with more bug fixes / security enhancements / other improvements via the AOSP common/generic kernel. It's entirely possible to move to a newer LTS branch. There are no closed source kernel drivers. Is it worth the time, when newer LTS branches have substantially more attack surface and tons of regressions that are going to need to be detected/fixed? Bear in mind it would not expand the lifetime of devices at the moment, time several hardware components won't receive more than 6 years of firmware support.
There are already people who have gotten the mainline 5.15 kernel working with the Pixel 6, but from 5.10 to 5.15 there are a lot of regressions, and there's a lot of new attack surface. There's a reason that ONLY the Pixel 6 among the Pixel family has been vulnerable to several serious core Linux kernel vulnerabilities disclosed in the past few months including the branded dirty pipe vulnerability. There are both advantages and disadvantages to using a newer LTS branch. Unfortunately, one of the disadvantages is that there are more bugs overall, including more vulnerabilities overall. Many software projects mature over time and the rate of finding vulnerabilities goes down. That's not the case for the Linux kernel. It's having vulnerabilities introduced at a faster pace than they're fixed. It isn't better from a security perspective to use the 5.15 LTS rather than the 5.10 LTS, especially with the additional changes backported by AOSP including security enhancements like mitigations, not just bug fixes. It may be a good idea to move to the new LTS branch once it has matured for 1-2 years, but definitely not months after release.
As stated earlier, there are no closed source kernel drivers for AOSP, GrapheneOS or even most mainstream Android devices running the stock OS.
The reason for using an LTS kernel branch with 6 years of support from kernel.org is stability. Porting forward the drivers to each new kernel release is entirely possible and isn't a lot of work when it's done incrementally. Not that many changes are even required. The issue is that there are substantial regressions in each Linux kernel release and it takes at least 4 months or more to get a production quality kernel for a specific hardware target with nothing break, the massive CTS/ITS/VTS passing, etc. Pixel 6 uses the Linux 5.10 kernel branch which was the latest at the time, and those LTS branches have 6 years of support from kernel.org and expanded support with more bug fixes / security enhancements / other improvements via the AOSP common/generic kernel. It's entirely possible to move to a newer LTS branch. There are no closed source kernel drivers. Is it worth the time, when newer LTS branches have substantially more attack surface and tons of regressions that are going to need to be detected/fixed? Bear in mind it would not expand the lifetime of devices at the moment, time several hardware components won't receive more than 6 years of firmware support.
There are already people who have gotten the mainline 5.15 kernel working with the Pixel 6, but from 5.10 to 5.15 there are a lot of regressions, and there's a lot of new attack surface. There's a reason that ONLY the Pixel 6 among the Pixel family has been vulnerable to several serious core Linux kernel vulnerabilities disclosed in the past few months including the branded dirty pipe vulnerability. There are both advantages and disadvantages to using a newer LTS branch. Unfortunately, one of the disadvantages is that there are more bugs overall, including more vulnerabilities overall. Many software projects mature over time and the rate of finding vulnerabilities goes down. That's not the case for the Linux kernel. It's having vulnerabilities introduced at a faster pace than they're fixed. It isn't better from a security perspective to use the 5.15 LTS rather than the 5.10 LTS, especially with the additional changes backported by AOSP including security enhancements like mitigations, not just bug fixes. It may be a good idea to move to the new LTS branch once it has matured for 1-2 years, but definitely not months after release.