All this enthusiasm for releasing capacity into Marylebone is forgetting that:
a) the last opportunity to switch over from the Chiltern up fasts to the Met up fasts is just south of Harrow-on-the Hill. So the Chltern lines from there to Neasden would be unused, after which the ex-High Wycombe traffic would take over to Marylebone
b) if a decision to provide a new crossover south of Wembly Park was provided it would involve a grade separated link because the Up Met is now on the far eastern side of the formation.
c) even if that expense was agreed, the two track railway that is the Met lines into Baker Street would then be carrying it's current load plus additional Met trains carrying what is the non-stop passenger load from beyond Amersham currently running into Marylebone.
Your little bit of crayon work would not only cost TfL dearly in both trains and infrastructure, - it would also overload the line from at least Wembley to Baker St. TfL couldn't buy that, even for a few more fares.