I'd be surprised if Halo 5 didn't at least match Halo 4's sales numbers off of brand recognition and marketing alone. That's not an argument for the quality of this game or a hypothetical sequel - I just think people like us who care about the minutiae of the game mechanics ultimately don't matter as far as sales are concerned. The average consumer takes little more than a cursory glance at the game compared to how we look at the game.
I think a better development alternative would be to go smaller, not bigger. Halo CE was made by like 20 people for a few million bucks. Halo 2 was made by like 60 people. Halo 5 doesn't have to cost a hundred million dollars, does it? If they spent less, they'd make even more back. They don't have to pander to the market leader's audience to be successful; they don't have to have the best graphics at the expense of other functionality; just accept that Halo is no longer top dog and focus developing a leaner and more focused game. They can keep what's left of their original hardcore audience and with the right marketing still get enough of the general public to stay profitable.
I think that's ignoring the fact that games are far more complex than they were. Putting aside the gobs and gobs and gobs spent on marketing... everything is far more detailed (a Reach assault rifle was as many polygons as an entire Marine in Halo 3... I think comparing Halo 4 to Halo CE would be mind-blowing), physics and scripting and AI are more complex, you hire orchestras and motion capture actors, networking is still a bitch, and then there's all the support/extra features, including the ones people are maligning for not being as good or better as previous games--Theater, Forge, File Sharing, etc. I don't think you'll ever be able to create a AAA game as cheaply or as quickly as you could back in CE's day, especially if you're starting from scratch. That time has past (although I also don't see any way that video games can continue ballooning in cost and becoming more baroque. Eventually that system is going to tank and burn.)
Secondly, Halo has never succeeded commercially due to a hardcore audience (especially if you're talking hardcore as in competitive players.) They have always marketed Halo with an eye towards capturing a broad segment of the market because that's where success lies.
Unfortunately, in the eyes of many forumgoers here, that broad segment shifted towards a different playstyle.
(Not arguing with your point about leaner and more effective development, though. Video games are expensive, but they don't have to be *that* expensive. I think at some point all those millions get you less in returns. Likewise, I think it's entirely possible for Halo to orient itself towards a hardcore audience and be very successful, albeit nowhere near as successful as previous games.)