Of course we appreciate users that ask good questions and/or provide good answers. We would like to keep those people around to continue adding good content. Regarding your specific examples:
Madhav Sudarshan is a mystery. I know they are not alone in joining, adding some good things, and then disappearing.
Victor Juliet was periodically banned for a reason. We want to retain users but only if they will play by the rules.
Ethan Hunt is still around, and I would not place him in the past tense. Yes, he had an initial flurry of questions and some had to be closed, and there was a bit of frustration from that. But he has asked some good questions since then. He has also been in chat (see below) and seems to be learning how things work here.
So as you can see, there are many reasons why users might leave. Maybe they run out of questions, or maybe they are just no longer interested. This isn't always a bad thing. You can't please everyone all of the time.
I'm not saying that we can't or shouldn't try to improve though. Suggestions are welcome.
One thing that may help is inviting more people to chat. It's probably not very clear to new users that it exists or how to get there. Even if they don't have any questions/answers for some time, hanging around in chat may help to keep them engaged more. As with general participation, some people just choose not to participate there, and that is fine too.
SE Aviation has a Chat Room - Come join us!
Another possibility is harsh response to questions/answers that are perceived as low quality. I think many people here try to be as polite and helpful as possible, but things can come across more negative in writing than intended. If there are any good examples that could be a good point for discussion.