But surely all you would need to do is to set other fares to
compensate? IMO this should have been factored in when they designed
the system: it's what most European systems do, and you can still get
transfare tickets in Newcastle, for example.

In any case, I am sure a lot of people will be making decisions based
on the cost. If travelling on the journey I gave as an example, I
would currently change at TCR for the Northern Line. If I could use
the bus for free, I would change at Holborn for a bus as I usually
reckon this is quicker. So no additional usage.