It seems we've got our wires crossed and we're talking about two different things.

Like you say, you can't use new packages that require newer kernel features that are not present in an old kernel. However, I understood we are talking about the reverse (old packages with new kernel). I thought that were discussing why Mer doesn't use a mainline kernel, thus I was explaining how the use of a new/mainline kernel would not prevent the use of the old packages provided by Mer.

It's worth noting that Mer doesn't actually include a kernel (or at least it wasn't included the last time I checked). The kernel and drivers fall under hardware adaptation which is considered as a separate project.