I don't think it does, and the BCE was based on the original estimated cost of the project. The original estimated cost was about half what TfL now believe the project will actually cost. That has a huge impact on whether the project is worthwhile.
That assumes that people want to go to Watford in the first place.
The extension would help connectivity for Aylesbury, Amersham, Rickmansworth and Chesham into Watford, definitely. But, unfortunately, that again isn't TfL's problem. That's a problem for Herts and Bucks councils. For people within Greater London the extension wouldn't really create any new links that aren't dealt with by Overground and London Midland services from Harrow and Wealdstone and Wembley Central. At its furthest the Met is about two miles from the WCML- it's less than 1.5 miles from Pinner to Hatch End, for instance- and that really isn't far enough to claim "new links".
I'm very disappointed that it's looking like the link will never be built. But I don't see how we can blame TfL for it. TfL were given a hospital pass by DafT and Herts CC, and now because London voted for a Labour mayor the Conservative government won't assist any further. I don't see how TfL can justify spending London taxpayer cash on a project that primarily benefits the people of Herts and Bucks.