Seems a 2-board kit would be no harder to implement; presumably, it would eliminate need for a mux board.
These upgrade kits are used for a lot more locos than just PS1s, and I'm sure non-PS1 upgrades are a major portion of revenue. In fact, I've never had a PS1 but have upgraded both whistle-only MTHs & locos of other mfrs. I'm not aware of any desire by MTH to limit upgrade kit sales to PS1. I'm sure the MTH Comptroller is very happy when they are used to upgrade Lionel, Williams, Weaver, etc. to DCS. In fact, one of the MTH executives told me that they are working on (didn't say how hard) a kit that won't need a flywheel just to permit bringing DCS to more non-MTH locos.
Although I may be totally beserkshire,* I still don't see the point of having one board for diesel and a completely different board for upgrade steam, with different lighting, and a third for factory steam. It certainly increases production costs.
*I shall some day resolve the Berk's problem, but that's another story.
I think I'll drop off the thread. What MTH has produced is what it is, and while I don't understand the logic, we have to live with it until MTH makes further changes. No point is discussing what might have been or should have been.
