We seem to bang this around a lot, it is almost as good as some of the other threads, many of which seem to end up getting heaved, with which manufacturer is evil, who stole the cheese and the like....
I don't know where the OP got the idea that Lionel was 'throwing it away' or something. From what I can tell about the new system, it is backwards compatible with anything out there they have, whether it is tmcc, legacy or lionchief/bluetooth. The only thing they seem to be removing is the 990, the 1l is still available (unlike MTH that totally dropped the remote, though the new base supports it via wired connection). When they created legacy, it supported TMCC, when MTH went to PS 3.0, it supports PS 2.0 engines.
The reason that 990 unit was listed at that ridiculous price is someone was playing games, figuring they could troll a sucker into buying it, as if the 990 was going to become this incredible collectors item, which is ridiculous, but that is the way some yo yo's think. About the only thing the 990 gives you is the bigger remote.......
Legacy and DCS are different than DCC in many ways, in large part because of the nature of 3 rail trains. Some will bristle at the notion, but things like the swinging bells and the chuffs and the steam whistle discharge and the other little things the engines in 3 rail gave us are in some ways more akin to the accessories that 3 rail has had, or the operating cars and the like (it isn't that these things aren't prototypical, they are, but they are in a sense more 'play like' than what you see in the world of HO and N; some of that is that 3 rail O has the size to implement those features).
So DCC focuses on sounds and being able to modify them and it has programmable settings (CV) that allow you to program the decoder to do things in a certain way. If you don't like the speed steps, you program the CV, if you want breaking to work differently, you reprogram it. In a sense, legacy/DCS is kind of like the applications that let you design a website visually, and it generates the code, vs programming in HTML to build the website yourself (rough analogy). DCC's biggest advantage it is open standard, so if you don't like the decoder that came with an engine, if you want a feature another decoder/system offers, you can pretty easily change it (is a standard plug these days), you aren't locked in to one manufacturer.
Only reason I could see lionel offering DCC on the new base would be if they think there is a market in 2 rail O, where some of the persons engines are DCC and some are converted Lionel legacy units from 3 to 2 rail. Thing is, though, in 2 rail if they convert an legacy engine from 3 rail to 2 rail, I suspect they would also convert to DCC *shrug*. The only other thing I could think it would offer is to control PS 3.0 engines using their dcc compatibility mode.