Id like to say you may be wrong there.
Back a few years ago there where at least 2 topics that went on and on about adding MP to TRS. Ideas where put out, ideas where shot down and it went on and on. For at least 20 pages in one of them. And at least once every few months a new topic on why MP should be added would be stared by some one new.
But this is not to say you are 100% wrong, I do think you are right in some regards. Most people have LOST interest, and for a number of reasons. Of which I'll point out. One of them is like Danny said,
This is true, its a big pain in the caboose. Lets look at MRL, though this could apply to UMR or any other of the large routes. MRL takes, at tracks speed's about 5-6 hours to get from the east end to the west end. Its about a 2 1/2 hour run from Livingston to Billings. If you where to make up a train in Livingston, which might take you an hour of switching to-do. Then run it to Billings, you've just spent 3-4 hours on a train. That's a huge chunk of time dedicated to one train. Now imagine that just as your pulling into Billings the server crash's. Now your 3-4 hours of hard work is gone. Now you have to rejoin and restart your train all over again. And this can be very discouraging for many.
The other thing I think that many people where disappointed in is how content is handled. Many though that if MP where added, they would be able to invite their friends into their routes and use what ever content they wanted. But seeing as that's not the case and every thing must be on the DLS, many have shied away.
And the rule that every thing must be built in or on the DLS is a double edged sword I think. On one had it means that if I make a route for MP, you dont have to go all over the web to site like RRMods, Jointed rail, VMD, TPR, to get the content. Its all in one easy to find place. But at the same time, Jointed Rail has some nice locomotives, in their freeware section, that I'd love to use but cant because they are not on the DLS.
Now there are a few things that could be done to improve the MP experience for all. Some are in N3V's hands, others are in our own hands.
One thing that N3V could do is make it so the session host could save a MP session and resume it latter. This way my 4 hours working a yard are not wasted when the servers go down. And this way, sessions could last much longer and be operated more prototypicaly. Example being, i could host a 4 hour long session once a week on MRL. At the end of the 4 hours I save the session. Then next week reload it and we all start off right where we left off. This would be the equivalent of running model trains and pulling the pug on the layout. When we plug it back in our trains are where we left them, and we just keep rolling.
The thing that we, the user's can do, is to provide top of the line quality content to the DLS. And that right there is a win win for every on. If some one where to make Jointed Rail quality locomotives and put them on the DLS, then MP sessions would have that high of quality locomotives in them. At least my sessions would. But even the non MP user's would get fantastic content.
Now I know for a fact that, at the time of writing this, I have no real skill in 3D modeling. So I try to contribute to the TRS MP community by making prototypical sessions, and high quality routes. For the time being my MRL route and sessions are on the back burner and I am focusing on two project routes. One of which is close to being announced and released. The other has been announced, but is no where near ready to be released.
Just my thoughts on this whole topic.