I fail to understand why having a more recent kernel version is actually desirable in this case. What is the benefit you perceive with having a more recent kernel in this case?
That being said devices originally released with Android 15 are required to use the latest LTS kernel version which is 6.6.
When it comes to existing phones with existing users, you are right, there is no visible value (ideally).
But when it comes to development (especially of new devices, both SW and HW), it has a lot of benefits. You can support more modern features, you can pick more modern hardware (with mainline support). I could list random notable changelog entries from the past year or two, but I guess you are familiar with them too.
(Beside I guess, you could ask the same question about this news item: what's the benefit of bumping the kernel for an existing device, which won't get any notable updates, beside the latest and greatest gmail client, and new telemetrics?)
I really wonder when will the Android kernel customizations be upstreamed enough that it won't be lagging this badly behind