The main problem with what you call MHS (and what I'd call state-of-the-art conventional fast train) is sharing tracks with 'normal' trains, which go even slower. Especially slow and long freight trains, which would have to make way mid-way along the route, yet accelerate slowly. So an upgrade-only option is really cheaper only if the capacity limit is not hit, and freight railroads won't protest. But once you build a new line, it makes little sense to not build one suited for 200 mph -- the WCML in Britain and similar projects have showed that trying to squeeze more tracks into existing infrastructure can be even more expensive. (BTW, given population numbers, Washington-Pittsburg would justify a real high-speed line.)

I suggest to you that there is a better reason to have both 'MHS' and 'VHS' than getting branchlines for trunk lines. Conventional fast trains could have more frequent stops , e.g. serve more stations, along the same corridors (even if not necessarily on the same lines), too.

_{*Lunatic*, n.
One whose delusions are out of fashion.}