I'm not sure I agree with this.
Changing established continuity is quite insulting when it's done on a grand scale. If I've seen, for example, Captain Smith's identical twin brother in multiple episodes I'm going to take issue if Captain Smith states a few seasons later he's an only child. Writers should respect their audience's intelligence. Changing established story material because it's inconvenient to the current storyline being written is incredibly lazy and an insult to those who have stuck with the series long enough to remember.
I think that's taking it rather personally, and it's also something of a straw man example. In the past, that sort of thing was done from time to time -- see Chuck Cunningham in Happy Days -- but that was when nobody expected old TV episodes to ever be released on home video or catalogued on Wikipedia, and so continuity wasn't as great a concern. Today, creators are more aware of audiences' regard for continuity, and so changes to canon are likely to be subtle when they do happen.
But there are still cases where large-scale changes to canon are considered appropriate. Dallas retconned an entire season as a dream in order to bring a popular character back from the dead, although I gather that wasn't a universally appreciated decision. Neil Blomkamp's Alien 5, if it ever gets made, will reportedly ignore Alien 3 and Alien Resurrection, just as Superman Returns ignored Superman III/IV. Sometimes the audience doesn't mind a change to the canon, or even welcomes it.
Bottom line, these aren't documentaries, they're stories. We're all just pretending any of it happened at all. And that means that if some part of it didn't really work that well, we can pretend it happened differently or didn't happen at all. Of course I'm not saying that it should be done willy-nilly; I'm just saying it's misunderstanding how fiction works to assume that "canon" means every last tiny detail is perpetually immutable.
I actually think your reasoning is a straw man argument to be honest. All of the examples you've given me of stuff that was exiled out of canon are movies that were not popular with the fans in the first place. As far as Dallas goes it was the definition of a jump the shark moment and the show never recovered from it. Bobby in the shower is still infamous to this day. Regarding Happy Days and other old shows you stated yourself that no one expected those shows to see the light of day again. It was a different era with different sensibilities.
It's 2015 now. We can access every episode and movie of Star Trek and pretty much everything else. Even if you don't remember every detail (and why should anyone be expected to anyway?) you're going to notice glaring continuity errors when they're staring you straight in the face. Not that every last little detail should be adhered to before a script goes before a camera but excising inconvenient continuity that gets in the way is a cheap and lazy method of writing in my opinion. If we're not supposed to buy into the "world" we're watching and only care about what happens from one episode to the next and continuity be damned then why should we become invested in Star Trek in the first place. Voyager's use of the reset button did that show no favours.
It's not that I take it personally. I just like to buy into the fiction I'm watching and if major details are changed it pulls me right out of it. I'm perfectly willing to allow for creative licence but sometimes it takes the biscuit. That's not to say I think Star Trek as a whole is guilty of this on a grand scale. I'm more disagreeing with your reasoning about why established "facts" can and should be changed if they get in the way.