Fascinating. I've been a passenger on the Met since infancy and in 60 years have seen the Victoria. Jubilee and DLR built and the Piccadilly extended, but this is the first extension of the Met. In fact, this must mark the first expansion since the great retreat of the 1930s from outermost Buckinghamshire.
The LNWR branch from Watford Jctn to Rickmansworth opened on the 01/10/1862; whereas the Watford Met branch was opened much later on the 31/10/1925. The LNWR (later LMS) and Met each wanted their own lines between Watford and Rickmansworth. true competition unlike todays railway!.
The common sense approach would have been for the LNWR branch to have been upgraded and linked to the MET.
By the time the link is completed it will have taken over 150 years for the folly of our ancestors to be finally resolved.