I know there are several seminal works locked in archives or even just lost.

I couldn't think of any specific examples off the top of my head, but I was considering the fate of Microprose, Sierra On-Line, and other studios that were gobbled up, disbanded, broken up, etc.

Your Mechwarrior example is a good example of licensing, where you might have defunct TTRPG studios (FASA) licensing a property to a have company it studio that has also gone though several mergers.

There should be a "use it or lose it" provision in copyright law, kind of like back in the day with what happened to "It's A Wonderful Life". The only reason IAWL became a Christmas classic isbecause it became public domain.