Well a couple things:
TMCC/Legacy locomotives are still run from a TMCC/Legacy base. If you use the MTH cable to connect the TIU to the TMCC/Legacy base the DCS remote will send commands to the TIU, which will in turn send commands to the TMCC/Legacy base to send to TMCC/Legacy locomotives. You still need the Lionel components to make it work.
The MTH locomotive I have that is now TMCC control was a conventional PS1 locomotive when I got it. I added TMCC boards to get command control into it. Many times it's cheaper to purchase an old conventional Lionel or MTH locomotive and upgrade them to TMCC or PS3 than to purchase a new locomotive. Or if someone has only one of the operating systems and really wants that system in a locomotive that is not equipped that way from the factory they can add/change the command system of their choice. The locomotive doesn't care what electronics are in it. A MTH locomotive will operate fine with TMCC electronics fitted in it, and a Lionel locomotive will operate fine with PS3 electronics fitted to it. Just a matter of board swapping or adding command control if you want. It's not rocket science.
Paragraph 1 is basic, and syncs with what I stated earlier. That was not the issue I was raising. This is about the operating systems, and the electronics in the engines, not about the manufacturer of the engines. My issue was about running MTH engines equipped with DCS on TMCC or Legacy systems. It can't be done. In fact, that has been a sore spot in the hobby for many years.
What you have described in paragraph 2 is essentially converting a DCS equipped engine into a TMCC or Legacy equipped engine. Such an engine would of course run on a TMCC or Legacy system. I would have no interest in that, and I strongly suspect it would interest very few people, but I get your point that it works for some.
Finally, I did love your line that "The locomotive doesn't care what electronics are in it." Totally agree, but the operating system sure cares a lot about what electronics are in the engine.
Pat