"Just curious but what does keeping the power and the signal separate have to do with adapting the system to a layout?"
With DCC you need to have the power and signal adequate in all areas of the layout. In 1994 or so, when TMCC was under development, this meant multiple power boosters per layout in many cases. With TMCC, the power was already there on your layout, and you just needed to supply the TMCC signal (command base). Very simple and adaptable compared with DCC at the time.
""decent quality sound (also not available for DCC 25-30 years ago)"
Not true. 18 years ago I had Soundtraxx sound only decoders and back then they were old tech. Originally they were released in 1998. The sound wasn't great but I would say it was decent."
Not to be too glib, but 1998 isn't 1994. There was no high quality DCC sound in the early to mid-1990s when TMCC and Railsounds were developed. Those who were familiar with the available sounds felt that Railsounds was much better sound than that available for DCC. Neil Young, who is renowned for being particular about sound quality, felt he could develop a better sound system than that which could be developed for DCC. Feel free to argue with him if you like 
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DCC is neither simple nor problem free all the time, if that's what you are stating. TMCC is very simple and usually problem free for most people. The relative lack of magazine articles about TMCC problems over the decades and the multitudes of DCC articles speaks to the complexity and reliability/ease of use of the two systems. Your mileage may no doubt vary, but hundreds of thousands of Lionel customers were and are perfectly happy with TMCC and Legacy (and LionChief).
Perhaps if you are a 2-railer you are living in a different universe and market space? Lionel is not addressing your needs, to be sure. They have largely kept their customers in mind, which included, back in 1995, 70+ years of three rail O gaugers with large collections of locos with open frame AC motors. That was their target market. There were no suitable DCC decoders then for that application. The power boosters available were expensive and not well suited, which explains why not even gifted individuals were adopting DCC for three rail O gauge. That happened after TMCC was developed.
