Robin May writes:

And partly that there are very few useful interchanges.

There are actually a few that are fairly good. Stratford...

I'm more familiar with the Western End, maybe the eastern end is
better in this respect. There are no useful interchanges between
Gunnersbury and Willesden Junction. Since I live in South Ealing it
is a a roundabout route to go into Turnham Green and then back out to
Gunnersbury, and usually involves 15 minute or more waits at both
Turnham Green and Gunnersbury. An interchange between the Piccadilly,
or even just the Ealing Broadway branch of the District would be
useful, as would an interchange with the Central line.