Good grief. So long as the MP and SP are two separate entities and don't rely upon one another (which I strongly suspect, if MP exists this will be the case), what does it matter?? Usually the teams and budgets for developing such aspects are separate entities, and asking questions like "why couldn't they have spent x money or resources on y?" are meaningless, because those resources, people, or money never would have been allocated for SP in the first place.
I can't imagine they would have had the same team working on MP and SP if they did make a MP for DAI. Honestly, when I heard ME3 had MP I was skeptical, and it turned into one of the funnest games I have ever played, and I played it for more than 2 years with friends of mine. If DAI has something similar that I will end up loving just as much (in its own way) as the SP campaign, then great! I've been saying for years that I would love to have the opportunity for a good cooperative RPG that isn't a freaking MMO. I know of many RPG fans who feel the same. Maybe Bioware will unlock that door, finally.
And I'm certain that *IF* MP exists, people will be able to play the SP campaign without ever touching it, and just pretend it doesn't exist, if that's how they want to play.