I think people like to say gameplay over graphics, but if it actually looked as bad as to not have shadows, they'd reconsider. The gameplay is not going to be wildly different to the 360 games, and if it's going to look as good as those with fewer AI, drastic cutbacks like no shadows or whatever, why not just keep playing the 360 games? That's too far to have to sacrifice for co op, there's surely a limit.
Not only that, it would be an embarrassment for 343 to release something that looks an entire generation or two old, not least on the biggest release for the console. Again, there are nowhere near as many people playing locally as there used to be, so it's not surprising. They're probably under a lot of pressure to just get singleplayer and multiplayer right after the MCC, or even just being a AAA game in the first place these days, so it's not a big focus. They have enough work and polishing to do.
Imagine if the next Smash Bros didn't have local multiplayer lol, and they said they couldn't do it because they couldn't get 60fps. The community would (rightfully) go nuts, they can't just hide behind some made up (or half-true) technical excuse.
I think that's an exaggerated example, the proportion of people on Smash Bros playing locally is completely different to the people who intend to play co op in Halo 5. And it's not a made up excuse at all, I don't doubt that they couldn't reach 60 frames, it's a surprise to me that they reached 60 frames in the first place. They don't even need an excuse, if they didn't want to do co op then that's their business. I don't blame them for not putting out a statement that says "fuck the fans, we didn't want to do it." This is probably the best way they could break the news. A lot of people love 60fps and wouldn't have played co op at all. I wouldn't know if that's more or less than those who just want co op, but they're winning some and losing some, not just losing fans.
I'm sure there are disappointed fans but the bottom line is that their number isn't enough to justify some kind of workaround for this game that's apparently built around 60fps.