Having been away from the Nova community for five years, I was both surprised and ... not ... surprised at the lack of TCs completed in that time, especially given the number that were seriously in development when I last checked. ISTR many of those had full-on development teams and forums supporting them, too.
I think the OP is pretty insightful. I was thinking about the TC/plugin thing a lot since I started playing EVN again. The thing about EVC was that it was small and simple and just begging to be added to with plugins, especially with obvious launch points like the Cydonian/Lethian conflict. For example, I wrote a simple addition to the universe that bolted on a bunch of systems themed around various friends I knew living in various far-flung corners of the world, with a couple of extra ships to fill in gaps and a storyline that involved discovering and befriending the (small number of) new systems. This was purely for my own use and enjoyment and didn't affect the original scenario at all, and it was very easy to do even given that I was running EVC on Basilisk II with no direct internet access and no third party utilities.
By contrast there are a bunch of things about Nova which make that sort of thing hard. Firstly, the Novaverse is old: it's hammered into us that this is an ancient spacegoing society and that therefore most of the stuff worth knowing/discovering has already been discovered/known even if it's since been forgotten, and any genuinely new discoveries will take a looong time (e.g. Kont probe launch, ascent of humanity stories). That makes "new discovery" plugins really hard to achieve.
Secondly, the Nova political story is balanced on a knife-edge, which makes adding plausible new factions without modifying the original story almost impossible. Related to this is the way the main Nova storylines are utterly dominating and world-changing: a pilot that completed one seems to have nothing left to play for - it's hard to feel motivated to continue playing when your pilot has, for example, become an all-powerful ascended being, and it's therefore very limiting and difficult to come up with a scenario which leads on from that (rather than being free to simply bolt on something which interests you). Plus it's hard to bolt on content which isn't affected by these major storylines in some way - I think plugin authors feel the need to write big Nova-quality storylines or at least make their plugs compatible with the major story threads simply because that's the paradigm that Nova established.
Thirdly and relatedly, the Novaverse is so complex and "closed" that trying to write a plugin which doesn't interfere with it requires a lot of work. To me it's always seemed like it'd be easier to write a small TC using Nova resources than a small plugin, just because then you're not constantly tripping over someone else's work.
Fourthly and again very closely related to the above, making a decent Nova plugin isn't as simple as firing up ResEdit anymore - you really need some sort of third party tools. MissionComputer is great but only works on the Mac, and not (I believe?) on OSX. EVNEW is also great but offers comparatively bare-bones functionality, and RezEditor is not exactly user-friendly (and wasn't around back when I was last here). No matter what package you use you have to worry about converting it to the other platform. It just asks a lot more of the developer.
Nova is a fantastic little engine for telling your own story, but the stock universe makes it pretty difficult to do so without a total re-invention, which means TC, which means a whole other ballgame. When my hard disk crashed and I lost my EVN install (and my only copy of that EVC plugin ), I didn't reinstall because I knew I had reached a point where I was only going to continue enjoying the game if I started modifying it myself. And the plugin route seemed no less work than the TC route. Now that I've come back to it I'm starting to feel that way again very quickly.
...that came out sounding a bit more pessimistic than I had intended.
Also, hello again Nova community.