For the last 12 or so years we've been paying $60 for a Halo game and with Halo 5 they have given us 60fps, HD resolutions, full single player, loads of art content, Warzone, MP, tons of customizations, theater mode, free content coming down the line, and so on. It isn't unrealistic for them to try to make a little bit of money off of microtransactions seeing as though the barrier for entry for the players has stayed the same but their costs and effort have only increased.
Halo 3 had next to no competition when it came out and it was the default FPS of the era. CoD is now that and Destiny seems to be crowding the full sci-fi gap. Games in general seem to have incredible staying power with incredible player retention. GTA V is a monster at the moment for securing and keeping a player base.
Sales will likely never be as huge as they were but the game will have a long tail. Halo CE had an incredibly long tail. It sold for years.