Lionel has some pretty smart people too. They developed Legacy and I doubt that they needed DCS to be inspired do it. I'm sure DCS is wonderful but giving it credit for bringing forth Legacy is a bit of a stretch.
As for the wiring issue, from my use of TMCC and Legacy, all I have needed to do was hook up one wire to my transformer’s ground. Works great. I never had to create a new ground wire around my layout.
Legacy control is backward compatible to TMCC. There is no need for Lionel to recall TMCC locomotives.
Enjoy the awesome power of DCS! However, talking down Legacy shouldn’t be necessary in order for you to enjoy DCS more.
What I think Shawn was trying to say is that due to the competition of DCS is what prompted Lionel to develop Legacy. Just for the record I am a MTH fan but not a DCS fan. I have heard stories about both DCS and TMCC having trouble working on layouts. You are lucky that TMCC works great on your layout and doesn't need any additional mods to make it work right.