The 'throwing away your medal' trope may be specifically American; it references a type of protest from the early 1970s in which returning Vietnam veterans threw out their medals (sometimes en mass) to protest the war. Media usually borrows it as a way to remind us that these are heroic warrior characters but they're fighting for the greater good and not for their corrupt/incompetent superiors (I'm sure you remember Wing Commander Academy had exactly the same scene in the finale.)
Yep, I do recall it in WCA. What struck me here was that unlike WCA - and unlike the historical precedents - here there was no indication of conflict or disagreement. But, I suppose it may have been meant to signal that his primary motivation remains his girlfriend rather than medals or promotions. That would make sense, I guess.
I've been thinking about just this lately. It's not just Babylon 5, it's something that happens to specific fandoms regularly. I've followed this sort of thing informally by watching the 'tracks' that the big sci-fi conventions run each year. Something like Dragon*Con always has a programming track devoted to Star Trek and Star Wars... and then the rest of a limited schedule is filled with tracks for the communities with the most clout that year. Beyond the evergreen Wars/Trek, there's usually two possible cases. In the first, a fandom remains strong for five to ten years after a finale or a movie and then finally fades: Stargate, Buffy, Firefly. But just as often it will disappear immediately after ending: Babylon 5, Space Above and Beyond, seaQuest, Battlestar Galactica.
I think that it has very much to do with the fandom's mindset at the end of a show. For Babylon 5, people felt bad about how it ended. We can debate until we're blue in the mouth the virtues of season 5, those last movies and Crusade... but at the time the reaction was 'meh.' B5 was a franchise that sold itself by convincing fans they were rebelling against the status quo to support something BETTER and that it would pay off in the end despite the other kids having better... sets, FX, actors, writers, etc. But the ending wasn't as satisfying as it needed to be for people at the time and while no one would outright say it, folks moved on. That's not to say "communities die when things are bad"... it has to do with how a community's group effort is focused. Space Above and Beyond has one of the most saisfying endings for any series and the show retains a high production quality all the way to the end... and I went to the 25th anniversary event and there were maybe 8 other fans there. But I think it's the same thing: the community in 1996 was wrapped up in a massive SAVE OUR SHOW campaign and when the result wasn't that the show was saved... people moved on.
Hmm, that is an interesting question. I'm sure you're right that things like "save our show" campaigns serve to galvanise fans to some extent, but I think ultimately, it comes down to the show, but not the show understood as a standalone product, but rather the show understood in its broader cultural context, In other words, not how good the show is, but how "special" the show is.
Now, it goes without saying, the quality of the show is also a huge factor, not just in the sense that the show has to be good to begin with, but in the sense that declining quality saps fan interest. This is what happened to SeaQuest, I think. Imagine, if SeaQuest had been canned after the first season rather than after the third. You'd be left with the memories of a show that was incredibly, incredibly unique - a mostly non-violent, science-oriented underwater exploration show (with Bob Ballard!) set in a more or less conceivable near future. But then the show dramatically changed in the second season, and again in the third, with a strong decline in quality to go with that. By the time the show was cancelled, I don't think it had a chance to retain a strong fandom, simply because so few people were left who genuinely loved the show. Most people liked it - they liked it enough to keep watching it week on week, but when it ended - oh, well.
But "specialness" is more important. Again, look at SeaQuest - it started off as its own thing, but by the time it ended, a lot of people were saying it's basically Star Trek under water now. So, why keep loving SeaQuest after it disappeared, when you can just shift your loyalties to on-going Star Trek shows? Something similar, I think, was the issue for Babylon 5. It's a show about a space station deep in space, where various races meet. Ok, cool - but it's going almost in parallel with Deep Space 9. Now, technically, Babylon 5's spinoffs outlived DS9, so it's not like its audience could switch from one to the other - but there wasn't that feeling that Babylon 5 is unique and special, and there's never going to be anything else like it. Combine that with a feeling of exhaustion - Babylon 5 did decline in quality, as you note - and you end up with a fanbase that's just not going to stick around for ages. I think the fact that the fan base exists at all today, and that periodically, there are attempts to revive the franchise, is already impressive enough.
Compare this to Firefly: it's not just that the show was short-lived enough that it never suffered a decline in quality. It's not just that the fanbase was upset about how it got cancelled - actually, many Firefly fans had no idea the show existed at the time it got cancelled, as in many cases, they only transitioned into Firefly later (typically: Buffy fans developing an interest in "this other show" Joss Whedon did). But Firefly was palpably "special" - for all the other space operas out there, there wasn't anything that was similar to Firefly. So, the show retained a strong following for a long time. Ironically, I think the reason people ended up drifting away from Firefly after a while was the film, which, though well-reviewed in terms of quality, plain and simply spoiled the franchise, both by making it less unique in story elements, and also by breaking up the crew in a way that prevented even fanfics from carrying on the story in an enjoyable way.