I can at least answer this
The original TTT server died because, after over a year of trying, we could not regain a stable population for a dying game. Many different changes were attempted and none of them were successful, and eventually the server was scrapped.
I honestly don't know how us having a gmod TTT server would go. Its around 6:30 rn, and as I look, I see about 3/4 of the current TTT servers are sitting at 0, and less than half of the ones with players have over 20. Now, my internet is shit right now, and I know yesterday when I looked, there were like 750 TTT server, and rn i'm only able to load about 400. But thats still a very high number of empty servers, considering that means that there are only about 20-40 servers that reach the same population as our CSGO TTT server.
However, your point about old regulars coming back still stands. Would that be enough to get our server on its feet? Maybe, honestly I couldn't tell you.
I guess it is up to the higher ups whether they want to spend the time/resources on this. I could see it going either way, although I'm still of the opinion that going back to GMOD probably isn't the best step for us.
Edit: And yes, you will argue that it is not a dying game. But we started having struggles with it around the 2012-2013 era, when it WAS dying (averaging less than 10000 players) and even when the population of the game itself got better, that didn't seem to increase the population on our server.