Your question makes a couple of incorrect assumptions, so let me clear those up real quick.
1. Splitscreen doesn't have any affect on FPS values unless you are actively using it. Meaning that those who use the entirety of their screen will have the 'clean experience' that you speak of regardless of whether splitscreen is included in the game. The only time splitscreen could even have an impact on the solo screen players is during online matchmaking, and even then it's not a framerate issue. The only obvious reason that they could have to remove splitscreen is to claim that their game will run 60 fps all the time, which is a marketing ploy at best. I've seen a couple of arguments out there that since H5 is being built around 60 FPS that using splitscreen would drop the FPS down to 10-15 making it unplayable, but that argument doesn't hold up at all. There are a few multiplayer games that lock solo screen multiplayer and singleplayer at min 60 FPS and lock splitscreen (both multiplayer and co-op) at a min of 30 FPS. There is no way that 343i, with the financial backing of Microsoft and the development team that they have, could not accomplish this with Halo 5. Especially since a good chunk of those games are indie games with a budget of under 100k USD.
2. Splitscreen has always been a factor in whether or not people buy Halo, but this issue (splitscreen being removed) has never come up before because every major Halo title prior to 5 has included splitscreen. Personally, I've been involved in debates over whether or not Halo is better than Battlefield, and Halo has always nosed just ahead due to the fact that it has splitscreen. 343i and M$ seem to think that Halo's major demographics are children who have no siblings, teenagers who have no friends, and young adults who have no roommates. Either that or they are pulling the whole "we're saying that we're going to take this one thing away and then give it back at the last second so it seems like we actually listen to you guys" ploy again.
3. I genuinely do not see how the desire to play with other people on one television and with one console qualifies as incompetence, which is what your second question implies. Going by the definition of the word incompetent, I would venture to say that a mutlimillion dollar corporation expecting people to buy multiple consoles and televisions so that all of their children, siblings, roommates, or friends can play with them simultaneously is rather incompetent. It would also be incompetent to actually go out and purchase additional consoles, televisions, and games to accommodate said children, siblings, roommates, and friends.
Now that's out of the way let me say this. There is literally no reason for you to be angry at those who want splitscreen in Halo 5, unless they are being abusive towards you personally. There is only one reason that a solo screen player would want splitscreen out of Halo 5, and that's the chance that they may pair up with a couple of people who are bad at Halo in matchmaking who also happen to be splitscreening. That's pretty much the only improvement you're going to see if you solo screen in Halo, but even that's not exactly the boon that it appears to be. Those same bad players will likely still be playing matchmaking, only they can't screen peek to help them boost your teams score despite their suckiness. Not to mention that you're just as likely to have those splitscreeners on the enemy team, giving you and your team a bit of an advantage. So overall it cancels itself out.