1. Just because DCS doesn't work for one person does not mean it's "unreliable". I've had 0 issues with DCS or TMCC. All I did in setup was read the instructions that came with my TIU, wired it how I was instructed to, and it has operated perfectly since day 1. Yes all you have to do with Legacy/TMCC is hook up one wire and you're good to go. Being that it uses radio signal there are people who have issues with the system getting interfered with by their house wiring or layout setup. So even Legacy/TMCC isn't a perfectly reliable system
2. The only reason DCS doesn't have the same things is because MTH never licensed it. They might now, and then there could be aftermarket products.
3. The only real "upgrade" path to Legacy is to buy a new Lionel locomotive. Lionel doesn't make Legacy upgrade kits available to the consumer. Want Legacy in an older locomotive? You have to make your own kit by ordering boards from Lionel and building your own wire harness. You can get older TMCC kits but in diesels you only get 4 RPM sound levels, steam needs extra hardware for 4 chuffs and puffing fan smoke. PS3 kits gets you all those features, plus remote couplers, out of the box without additional parts. Plus you get a locomotive that has the same features as a brand new PS3 locomotive out of the box. In addition there are hundreds of sound files that can be added to a PS3 locomotive, plus existing files can be modified and by the end user with programs like ADPCM. ERR upgrades give you limited files, nothing locomotive specific, and you can't change them. As far as Legacy Railsounds being the best around that's debatable. There are sounds in DCC that blow Railsounds out of the water.
I own PS2, PS3, TMCC, and Legacy locomotives. Quite honestly for those who want to operate their trains in a prototypical manner with the correct sounds (whistles/horns/bells) or be able to change them when they want DCS is the better, more advanced system.
WRT 1: You are correct. One person's problems aren't enough. But that's not the case with DCS. Many, many modelers (some very well known) have had trouble getting it to work consistently or work at all. Five TIU versions and dozens of software upgrades in 20 years - doesn't look like a reliable system to me (and I built reliable systems for a living). Over on the DCS sub-forum you have a legitimate rocket scientist (PhD EE from NASA Jet Propulsion Laboratory) who's had to diagnose and fix a number of DCS issues. Doesn't sound like a case of simple user error or reliability, does it?
2. Yeah, but MTH didn't do that, did they?
3. As I have said above, I wish Lionel did license Legacy and did offer easy upgrades. They should, but that's not saying they will. Who knows? Maybe now they will.