The reason Mer uses old packages does not relate to the use of Android kernels and drivers but actually relates to tivoisation. There should be very few userland packages, if any at all, that are tied to a particular old kernel. Linus' number one rule for kernel development is "Don't break userspace," therefore old packages should be 100% ABI compatible with modern kernels.

Mer developers have previously stated that they chose to use out of date packages (which are unsupported upstream), as they are the last versions that are licensed under the GPL v2 which does not have the anti-tivoisation clauses introduced in the GPL v3. By keeping everything at GPL v2, it allows Mer to be marketed to many more vendors, those who have a problem with disclosing source code.

The truth of the matter is that Mer becomes useful when tivoisation is valued above stability and security. It's a sad state of affairs and is one of the main reasons I've struggled to warm to Sailfish.