They were most definitely the junior club in Manchester until they were rebuilt in the 50s. With the solid arguments and research of the historian's craft, Gary James has established this.
But of course there are ignorant people who actually think that English football was invented when the satellite tv dish became a standard household acquisition (which just happens to correspond, conveniently, to United's period of utter dominance up until about 2011). Or other ignorant people who think that English football was invented in the 1970s, when the Liverpool juggernaut got going.
It wasn't. It just wasn't. I wonder if there's a single United or Liverpool fan under the age of forty who could tell you without looking it up who the first team to do the double in the twentieth century was (and it was very late on, by the way). Or that for quite a long time Aston Villa held the record for the greatest number of F.A. Cup wins. I'm not banging the MCFC drum here, and it's not a question of ‘liking’ or ‘disliking’ this or that club. It's just a question of having a clear sense of what football history, in its modern, codified form, actually is.