It might (or might not) be less relevant, but it's also to a great extent already-expended effort. Arm kernels have had a built-in decompressor for over a decad

It might (or might not) be less relevant, but it's also to a great extent already-expended effort. Arm kernels have had a built-in decompressor for over a decade, so the bulk of the effort went in many years back when the gains were clear and worthwhile, and now while there's some maintenance cost to keeping it rather than just ripping it out, it's not a big cost. And dropping the decompression has the catastrophically huge cost of breaking an enormous number of already-deployed systems when they update to a decompressor-less kernel. Incidentally, aarch64 kernels do not include a decompressor -- if you want to compress the kernel image then it is the job of the bootloader to uncompress it. Different primary market, different time, different constraints, opportunity to choose differently, different outcome...