This is all good in theory, but it only really works with 100% sign up, and let's be frank, you're not going to get that. Moreover, sometimes the assets are key to the storyline without being DeM devices. The best example I can give you is, of course, one from TI, since that's the mod I'm most familiar with. I could see some of our ships getting put into a pool like this*, especially if it was a true pool, with beneficial give and take. But I'll pretty much guarantee the Aesir ships wont be in such a pool because they're the ingame representation of the species that form the core of the TI storyline. Releasing them to be used for other purposes completely dilutes their impact in TI if they're used first by another campaign. Similarly, while I can't be certain of this, I doubt that you guys on the BP team would be keen to release your moon surface models, or your custom Sol fleet, for basically the same reason. And then there're other hero ships, like BWO's Golgotha, that are at the core of a campaigns identity. Keeping these exclusive is, in my opinion, a good thing, because it gives you moments in a campaign that are, essentially, once in a lifetime. Imagine if the vessel that rescued you at Enif Station hadn't been the Collosus, but a Hades, which we'd all seen in Silent threat. Wouldn't that have diminished that moment? If the shivan destroyer that took out the Lysander had been a Demon? Or the superdestroyer that destroyed the Tatenen had been a lucifer? All of those moments would always have been cool, but they were also all immensely improved by the existence of brand new, never before seen ships, and as such, they only really work at their full potential once. I want that moment in TI when you see a Xolotl unleash its fury for the first time. I'm sure I'll get that feeling one day when I see the Golgotha's main beam for the first time, or when I first get to skim across the surface of the moon in WiH. IMO, the creators of those mods deserve to decide who gets the opportunity to utilize it for that once in a lifetime moment, and that it happens in a project they care about.
That said, there are dozens, if not hundreds, of really good mods that are released publically, and, as a general rule, assets in released projects are traditionally considered to be free to use, so while they are locked up, they are also (eventually) released. Dead projects as a general rule do also release their stuff. Sure, there're examples of classic lost models (Dark's stuff is one of the better examples), but there're also really great situations, like Aldo's releasing of the Reci and LS ships, or the 158th modeldump (admittedly rescinded when the project restarted, but they were released when the mod was apparently dead, so the point does still stand).
One thing I do quite strongly oppose is private conversions of publically released models, which is why I applauded Droid's release of previously exclusive conversions of Stratcomm's Vasudan fleet, and also why I released my own conversion of Aldo's PVF Nefertum. But these are a specific class of mods, largely separate from what's being discussed here.
*I can only, of course, speak for the mods I've made, not the ones made by other TI contributors, who would, of course, have to decide for themselves.