You can only fully cap 3 at a time.
The total cap of desynth levels can be spread out over all 8, although one of them will get its level reduced whenever you go over the total cap.
As for the change itself, it doesn't really surprise me that they went with a low effort change. They haven't really done anything with desynthesis since ARR, with its useability popping up every now and then after that (glass fibers in HW).