David Howdon wrote:

It may also be because transferring the entire commuter load from
the Picc onto the Chiltern service would not be that practical.

It's impratical mainly because of infrastructure constraints - pathing
over the link line from Neasden South Junction to Northolt Junction is
very tight, with the various stoppers/semis and fast trains from
Stratford and Birmingham all competing for track space. A decent
service can only be had by restoring the through lines at at least one
of the original stations along the stretch.

If the infrastructure were available, running a more intensive stopping
service would only depend on ridership.