As a Man City fan who lurks on here, let me add to this. Between the start of the 20th century and the beginning of WW2, we had bigger gates than our out-of-city rivals in around three-quarters of the seasons and finished higher in the league in more than two-thirds. There's no doubt which was the bigger club at that point, and it wasn't them. Nor were they close to being on an equal footing.
In addition to the three trophies we won, we had two losing FA Cup finals and the odd unsuccessful tilt at the league title. We'd also set a number of crowd records, whereas their support was generally poor in relative terms. Man United simply had one purple patch, funded by huge (by the standards of the time) funding from a sugar-daddy between 1908 and 1911. To do that, they nabbed four of our players, two of them being their star performer and their top scorer, when the FA forced us to sell our entire squad as punishment for an illegal payments scandal. We poached their manager in 1912, though, and they never really threatened to enjoy any success again pre-Busby.
Then it turned on its head, but one significant factor that people usually miss was them sharing Maine Road while Old Trafford was unusable owing to bomb damage sustained in the War. They arrived heavily in debt, and went back a massive (by the standards of the time) £100K in the black, with lots of fans who'd started to watch both teams during their time at our place following them back to theirs when they returned in 1949. I've always said, only half-joking, that when they asked to share with us, with hindsight we should've told them to piss off to Salford RL. That's probably where they'd otherwise have had to go, and Busby probably wouldn't have joined them in 1945 had they been playing there.
I appreciate that no one else on here really gives a toss about this, so sorry for rambling on. As it's my team and the issue came up, I thought I'd add my bit. As for the actual topic of the thread, of course I'm an outsider but my impression has always been that it's extremely surprising that one of the Blades and the S6 porkers hasn't managed to become an established top-flight side and really become the dominant team in Sheffield. I suppose the oinkers had it in their hands in the 1990s and blew it.
As someone with a degree of affection for the Blades, I'm glad they did. But I'm sorry that the Blades had great chances in 1975, 2007 and 2020 to really become the major force in Sheffield and also let it slip. I don't know enough about why that was the case, but read threads like this with interest. It's always instructive to know what a team's own fans think. There's always a lot of stuff you never pick up as an outsider.