I think you're making two different arguments really.
One, you think parents should stay under the same roof, as long as they get on ok, even if neither of them is happy.
Two, you think parents leave relationships with their dcs' other parents so they can pursue new relationships. New relationships, blended families etc can be mismanaged or just not work and that is damaging for the dcs.
You could argue the second without the first and I would definitely agree with you. I do think that when parents leave a LTR to pursue new relationships, it can be messy and blended families sound like a total minefield. But plenty of people separate and don't look for new partners at all. Some people separate and find new partners, but it has little impact on dcs, as they never blend or move in together.
I think the first point about staying in a marriage where one or both parties are unhappy, at a low level, non abusive way, is trickier. I think that being in an unhappy marriage can make a person very depressed and lifeless. That is not good for dcs to see.
But, I do think that if people leave marriages because they are hell bent on finding prince or princess charming, at all costs, no that isn't good for the dcs. I think if you're planning this, to a point, you have to plan to be a single parent, not plan to be the future Mrs Charming!
Not to be too fiscally minded as well, but you would need to plan to basically run two household instead of one, so you would need ducks in a row financially speaking.
Also, have you tried couples counselling? I might give that a bash first, but depends on the couple.