I believe this happens because there is no real unifying force other than good feelings. In a company, people are forced to make reasonable efforts to work together or they imperil their paycheck. In the open-source world, people won't volunteer their time to work on your young project if it doesn't fit their taste profile quite exactly. Since it's rare for that to happen, lots of people spend lots of time re-inventing the same basic stuff and very few people spend time working on the difficult 20% of problems that would yield an 80% improvement in overall experience.
We see this over and over in the open-source world, and it's not without its benefits, but particularly in core underlying components like display servers, it can be very frustrating because you just want everyone to adopt something and move forward. The delays caused by the fragmentation can impair things for years as driver vendors play a "wait and see" game and talented developers look at it and say "Wow, that looks like a mess, I'll stick with Windows for now".
Personally I think Mir is straight up power grab. Canonical said "Why should we be bound by what these Wayland dweebs say? Even if their stuff is great, we are the biggest desktop Ubuntu distribution and we want to be in control. We will make our own display server." It is frustrating on the outside, but I guess you can't argue that it's not in Canonical's interests, and they may indeed end up winning the war (though it didn't work that way with Upstart).
I really don't fault them too much for their decision, especially depending on how much, or not their input was considered regarding Wayland... I'm also not sure when each respective project was started.
I do find a few quirks around Ubuntu... my HTPC box is about the only Ubuntu UI I work with, the rest are VM images running a server version. The intel audio (via hdmi) often doesn't come back after sleep/suspend, there are other issues there (i3-5010U nuc), but for the most part, I appreciate the Unity ui. That said, I don't do much beyond launch Kodi or Chrome and the terminal to fix problems after updates more often than I'd like.
Also, sometimes you need a dissenting opinion in order to make something better. As much as I've disliked some of the delays from MS wrt browser enhancements, a lot of the time what comes out after is arguably better than what came before... although I do still feel that MS should have just adopted SQLite for web-sql like everyone else.
It really does depend. In the end, canonical is dedicating developer resources to make something better, and is a much smaller company with far fewer resources than Intel.
I'm honestly astonished at how quickly people jump on to wayland to complain about it, about how things are changing, about how it's not perfect etc. Wayland is an ongoing answer to a plethora of long term problems that Linux is facing. Display-level app sandboxing, screenlock security, high DPI (scaling) support, etc. Dropping support for 30 years of legacy that is no longer relevant is just a bonus.
In the real world, things like multi pointer never worked (neither GTK nor Qt really supports it). High DPI monitors on the other hand are getting more and more common. So yeah, priorities.
And the wayland devs aren't some cabal out to prevent people from implementing those things you mention; or creating a solid plugin interface. They're just lacking in manpower because 1. it's hard and 2. few are seriously paying for that development to happen.
So when a company like Canonical decides to do their own version of wayland, not only are they actively removing potential users from the display server, and duplicating the migration effort, but they're also impeding development on wayland itself. So if you're annoyed multipointer isn't supported in Wayland, I can point you towards whose fault it is.