I think you're 100% right for the vast majority of people. People have less time nowadays, and millenials in particular want to have something akin to instant gratification. I had a kid at a convention tell me that he thought half an hour was a LONG TIME for a game. Console games take away a lot of wargaming's audience. Euro games take another chunk.
I'm a millennial. I'm willing to sit down for three hours or more for an engaging experience, but what tries my attention span are complex calculations that break me out of the immersion. I'm playing games for fun, not to do homework. I know other folks my generation and younger who are more than willing to sit for a 4+ hour game of 8-player Game of Thrones board game... or that long for Twilight Imperium's myriad mechanics.
I'd wager the reason modern games, eurogames and consoles are drawing audiences away from wargames is because they can have the same amount of fun without the cognitive loads of itemized rules and complicated CRTs inherent in older classic wargames. They can also be completed in far less time.
Again, I appreciate what you're saying, but for me it's just the opposite. When I think of great games, with lots of replayability, I'm thinking Terrible Swift Sword, Atlantic Wall, War in Europe, etc. Fed Commander, for me, is a step up from Battleship, maybe two. I just have zero interest in it.
Considering I haven't seen these titles in any of the conventions I've attended, nor did they appear in the stint I served working in a board gaming store, I'd guess the mainstream gaming community does not share your view. I and others figured classic wargaming was niche in an already niche hobby.
That said I'd love to get GDW's Imperium back to the table someday.
I could, fairly easily, fix SFB, at least to my satisfaction. I still don't think it would have any mass appeal, however, and as I've said upstream, I can't find people locally who will play it. As for Fed Commander, ADB says it's popular, but I have my doubts. I never see it in stores or being played at cons, but that could be for a wide variety of reasons. The only people I ever see discussing it are people who apparently get free copies for playtesting and the like. I have to believe that they sell more than that, unless they're making all their money on Shapeways nowadays.
If I were to set out doing a trek space battle game with the spirit SFB accomplishes, I'd dump classic SFB entirely and rebuild the game from the ground up, taking pointers from modern games.