I mean, forget live filter layers (they are fun and incredibly useful in Affinity Photo). But adjustment layers?
You can make an argument that GIMP users have learned to get by without them, but it's hard to make the argument that the wider graphics editing community doesn't see the need.
It is quietly delusional.
I always advocate to deliver a dependable app (fixing bugs, keep it in maintainable state) before adding nice functionality which always raises the risk of breaking something.
Affinity products are dirt cheap (for the average hacker news reader), so choosing GIMP or Photo is mostly no matter of cost, but of other factors. As even Affinity struggles with resources (to fix bugs) despite selling tons of licenses, how should the GIMP developers stem such an challanging feature?
And this isn't just "nice functionality" -- it's absolutely core to the compositing model. This sort of thing is built into operating system graphics layers now, and there are other, newer apps on Linux (Krita for one, I think) that can do it.
This is all the stuff GIMP users are promised for the future when the new GEGL gets integrated. But that's just like Infrastructure Week: it never arrives.
I'm not saying it is easy to implement; I don't know where I would personally start on much of this stuff. But I do think that it is indicative of the GIMP project's peculiar focus on scripting that something as crucial as the compositing engine in a photo editing application is routinely outsourced to another project.
It's such a basic function in other applications that it has to be holding back donations from interested users.
(I'm not sure where you're using Affinity Photo but on the Mac it has crashed on me about a dozen times since 2015. It does not strike me as more bug-ridden than GIMP!)