Halo has always tried to be more than that it seems. They go for this larger-than-life, epic struggle for the fate of all mankind type of thing, and a lot of fans of the series early on cared just as much about the campaign as they did about the multiplayer.
Nowadays you have more of a split. You have the hardcore MP guys that got into Halo's multiplayer so much in the early days that they became very competitive at it, and they seriously dislike how watered down it has become since Halo 2 and 3. Most I've talked to still hold Halo 2 up as being the pinnacle of true competitiveness in the MP space for the series (at least as far as console players go.... a lot of PC players prefered CE and all the modding that was done to it).
Then you have the guys that are into the lore and canon and like the campaign. Many of the afore-mentioned MP guys don't touch the campaign much, if at all. So if the MP isn't up to snuff (which arguably in Reach and H4 it really wasn't because it tried to be too many things that it isn't), then they say the series has gone to shit. The campaign guys on the other hand, much like yourself, may have enjoyed H4's campaign. But then again, there is even a split among the campaign camp because with something like the SP experience, beauty is much more in the eye of the beholder. Multiplayer can be much more easily parsed down as far as what works and what doesn't, but the campaign is at heart an artistic endeavor, aiming to entertain you but at the same time make you feel something about/for the characters. In that sense there are many more shades of grey as far as what's great and what's not when it comes to the campaign.
I don't think the Halo IP can survive on campaign alone, due to my afore-mentioned discussion of people wanting instant gratification more often than not these days. However, I also don't think it can survive on the MP alone for the same reason. If it keeps changing and pushing more and more toward the instant gratification side in the MP, it will lose all sense of its own identity and will cease to be Halo (some say this already happened with Reach and H4... I'm not sure).