Other Return to Ostagar deleted conversations also suggest Loghain and Alistair could be in the party together: Here, here, and here.
I imagine it was removed because then the right answer becomes: get all companions, persuade Alistair to back off. Similarly to why most people will never really consider killing Sten (albeit indirectly) or Zevran, or turning down Leliana.
I think the just wanted to force you to make choices that had ramifications. They are oh so big on that. Romance Alistair and you see how what a royal PITA their ideas of choices and outcomes are. Limited world view I say. Narrowing options. Cutting out what could have been some of the best options available. But they like to do this. They like to force players into corners. Very much so with Alistair.
I've killed Zevran many times and I haven't bothered getting sten in more games than I can remember. Only got him in the first few then found I just didn't like him. After DA2 hated the quinari and let him die - same as killing him. I've read posts from a fair amount of people that they don't recruit Zevran or Sten. You don't have to. You get Oghren and Morrigan and Alistair. You can pick from the others. You don't need all of them. They just wanted to make it an either or with ramifications that don't makes sense when you get to the 'you get to die for killing the archdemon' reveal. Biggest problem there is that it is some really terrible writing. I mean really terrible. I mean there are no words for just how bad it is because of all the opportunities had to make it clear that Loghain should not be killed... because of how daft they are about the whole secrecy of the joining when in fact once Riordan shows up it seems like the cat is out of the bag. Anora knows it could kill. We wonder if maybe the woman at the chantry who didn't want to let Alistair goes might have known. Even in awakening you have dialogue that makes the secret far less secret. Oghren wants to gargle and spit. That suggest he has a damn good clue about it. And that is really bad writing... to have this rule about the secrecy of the joining that is the very thing that so many twists and turns in your personal story hinge on only to then become quite lazy about the secrecy later on as the game progresses and even in follow up DLC. Anora should not know it is often fatal. How does she know? And Riordan discusses all of this in the open at the landsmeet but manages to keep out the very most important detail... the very detail that might have kept loghain from retreating at ostagar.... the whole story hinges on this and yet, it is done so poorly that one has to wonder if the writers just never sat back and examined the idea they were putting forth because it does not hold up under scrutiny. The justification makes sense but only so far which is really not that far at all. Simply put, their secret cost them the death of a king, civil war, death of all the wardens, probably death of a general, and oh yeah, most of the country blighted. They needed a catch. I get it. But they did a poor job with it. It was easy for them to make this joining secret but it fails in the long haul and epically so looking far more like so stupid uber secret club initiation than anything else.
Then they cover up the whole death by killing the archdemon bit with some more bad writing later when Riordan finally thinks to mention the death consequence where you just happen to get an offer from morrigan... oh what perfect timing. All of that is horrible writing. Or maybe lazy... I can't tell anymore. And yes, I guess it boils down to them not wanting you to have too many options but again, it's really done very poorly considering we can magically persuade the landsmeet to put us on the throne if we are nobles and we can choose who rules, but we cannot persuade Alistair so we get one last warden and have a happy ending if we want without having to screw the shrew.
Riordan's taint apparently made him dumb as a rock to not pull the warden and alistair aside at the landsmeet. I guess brain damage is one of the side effects of the joining....