However, let us look at this from Chiltern's viewpoint. By stealth
Chiltern have acquired a main line from London to Birmingham. The
development potential on that line is immense. Chiltern's investment
thus far is paying off. They have a loyal and satisfied customer base.

As indeed they do on the Aylesbury line. The fact that they can now run (and
fill) trains which run non-stop between Great Missenden and Marylebone (and
that quite a number of former LUL passengers drive to Great Missenden to use
these, in preference to LUL Amersham services) is testament to that.

Do Chiltern not also run services that run non-stop between Amersham
and Marylebone?

Moreover, TfL's electrification is wasted on Chiltern's diesel fleet.
I would suggest there is room for some strategic planning here. If
Chiltern lost the Aylesbury route it could be a gain for them.

From over fifteen years commuting experience on the line, my feelings are
the opposite. I strongly feel that the solution should be for LUL to operate
the Uxbridge and Watford (Junction, ASAP) branches, with the current fast
lines to Amersham and beyond transferred to Network Rail control, where they
can be upgraded to proper main line standards of minimum 75mph, as opposed
to the present stagger. The two systems would then be entirely segregated.

But wouldn't Chiltern then run into capacity problems south of
Neasden?

Maybe the line would need to be quadrupled there, although I doubt
there's space. IIRC, in the western entrance to Marylebone station,
there are photos showing the construction of two tunnels from
Marylebone to Finchley Road (ish), though only one is now in use - was
the second tunnel ever completed, and if so does it still exist?

The arguments about through running to the City are spurious. When I first
started commuting, all off-peak services terminated at Baker Street in any
case and it is only in recent years that through running throughout the day
has recommenced.

I believe the current situation is that all off-peak Watford/Amersham
trains now start and finish at Baker Street.

In spite of that, the numbers of passengers that do travel
through off-peak is negligible and could easily be catered for by a
cross-platform transfer to a Watford or Uxbridge service at
Harrow-on-the-Hill.

It's not cross-platform, unless some major remodelling goes on at
Harrow...

In any case, the through-running-to-City problems would be largely
solved by Chiltern's proposal for an enhanced West Hampstead
interchange.