That sounds like a story about the original 350/2 lease that got mangled. I’m not even convinced the original story is true, given that the 172s were bought at the same time, by the same ROSCO and they are not being replaced.
I really don't know... but it doesn't make sense to me that WMT would choose to get rid 37 of 87 of some of the most reliable EMUs in the country, and even if that can be explained it makes even less sense that the choice to replace the subfleet which happens owned by a different ROSCO would be a complete coincidence.
One possible solution (and
this is speculation!!!) is that Porterbrook thought they could persuade either SWR or TPE to lease them at a higher cost than London Midland were before and order fewer new-build trains, in a similar way to the stunt they tried to pull with the 323s a few years ago. However the 172s would be less attractive because the only other operator of them is Chiltern, and they weren't/aren't looking to expand their fleet. If this is the case then maybe they'll come whimpering back to WMT and offer them back...