Consider the physical world. Cars, toilets, old-school dial telephones, doors, blenders, paper books, hammers, bulldozers, can openers... have vastly disparate user interfaces, but no one complains about having to learn them all, nor do they expect some kind of chimeric "common user interface" and complain when the hammer and the table saw don't work the same way.
Hammers that required you to adjust screen sliders to adjust the force and angle of the blow would suck. Hammers and table saws that were given some sort of half-assed "common user interface" would suck bad.
So, yeah, there are a lot of bad UIs. That doesn't make (current favorite UI) a good one, though. The way that this gets better in the long term is for people to keep making new UIs (most of which will inevitably be bad). Trying to stop that process just means that it'll take that much longer before we get good ones.