I don't think that this is a limitation of Alien Dalvik itself, but rather an outcome of having only binaries.

IIRC, part of Alien Dalvik on SailfishOS is a kernel module - if we had the sources, we could build it for newer kernel versions or use something like DKMS, but as we only have the binary, we are limited to one kernel version.

On top of that, I've read somewhere (if I remember it right) that porting Alien Dalvik is comparable to porting SailfishOS itself, as they are many device-specific problems (like sensors), so that's another limitation of having it only in binary form, not just the kernel version limitation.