they never have been the "united" its only a very arrogant view that they are the only united that matters
not even the first ones (sheffield)
I'm sorry it sounds arrogant, but it is also a fact that Manchester United are the biggest "United" football club in the world. Certainly in terms of support, and a global brand football club. So, if "United" is mentioned then pretty much everyone knows what football club we are talking about. Even people that joke "who, Sheffield United, Newcastle United" as a response, are only doing so because they knew exactly what the United in the subject was Manchester United.
I'm not sure about the comment "the only United that matters", in terms of what you mean? To many fans, you could argue that their club is the only one that matters, and perhaps their biggest rivals. As a Man Utd fan, I couldn't really care less about Sutton United, they might as well not exist to me. That is not me saying they shouldn't, as they will have many loyal passionate fans who love their club just like other fans love their own clubs. But, it is all about context. If the back page of a national paper said "Ugly United" or "United Score 4", then I automatically assume it is about Manchester United. If the same headlines were made in the local Sutton rag, then I might assume it is Sutton United they are talking about.
At any rate, it is not just United. I'm pretty sure if "City" is used, people now generally expect the subject to be Manchester City due to their sustained success over the last decade.