Exactly. What I see is that the SoC provider just freezes everything at a given version and supports just that. For example I am currently building Android 9 on a QCOM SoC with a 4.9 Kernel. I don't think it will receive any future update...
So how did OpenWRT manage to build firmware with up to date components for it? The Qualcomm chips inside of it seem fairly modern for such an old kernel.
OpenWRT doesn't guarantee support of all hardware. I have a router flashed to a certain version with a newer kernel, and the Wifi doesn't work because of no driver available.
Coincidentally, this very router's OpenWRT isn't as fast as the manufacturer's firmware because it doesn't have proper drivers for the hardware NAT, so it has to do it in software.
Qualcomm networking chips mostly have open-source drivers, which are mostly upstreamed to the mainline Linux kernel releases. So there's nothing really holding back kernel versions on that hardware, other than the regular maintenance burden of keeping a Linux distro up to date—which few OEMs are interested in doing. Broadcom by contrast is less friendly to open-source, and their closed-source drivers can preclude kernel updates.
Note that openwrt has a big community of contributors and not all devices/features are supported. In contrast the manufacturer firmware is at least feature complete and easy for regular users to set up.
OpenWrt is also free. Both as free software, and free of cost. When you're paying a manufacturer for a product, surely it's not too much to expect them to ship with functional software that also happens to be up-to-date and secure?
You can get that, but not at consumer-grade router prices. I have a separate router that I put behind my stand-alone cable modem. I paid for that separate router about $200.00. And another $100 for the modem. A wifi access point cost me another $100.
So it's about $400.00 for a router that has updated firmware(pfSense). Or you can cheap out and spend only $100.00. This is what you get by doing that.
Support varies, you should purchase devices that include hardware which is supported by the Open Source drivers (even if you have to compromise and it still uses some small blobs that are free to distribute).
You should also purchase a device that includes enough storage space and RAM to support more than the bare minimum; that will help keep things future proof.
