The MCU is making the same mistake that comics make regarding "Continuity" (which is why the word can get a bad reputation in both fields). And I don't quite understand why. To me, the solution is simple: You should tell each story as if it is an entirely new story. It shouldn't matter if a character is introduced to us for the first time here, or if they have been in something before. At the same time, you don't need to do everyone's origins or use giant piles of expository to "fill the audience in". You just need to have everything they need to know exist within the framework of this story. Having seen other stories with the characters or situations should be a perk, not a requirement.
I really can't agree with this. To me, some level of continuity is important. If a thing has been previously established in the same story framework, it should stay the same – at least in broad strokes – or the differences should be explained. If regular Asgardians have previously been shown to easily kick a truck across a parking lot, Asgardians should not be shown with regular human strength in another movie. If the Kingpin has previously been shown as strongly dedicated to his wife Vanessa, he should not be shown next as a philandering womanizer – at least not without a good explanation, such as Vanessa no longer being in the picture and him trying to fill the hole she left.
That said, you don't have to
use all the continuity. If Kingpin isn't involved in a story, his romantic involvements or lack thereof are not relevant. Writing someone out and replacing them with someone else is usually a better option than changing them. Maybe the Rose, or Tombstone, or Mr. Negative are running organized crime in NY these days – that's a better option than redefining who the Kingpin is.
I do agree on one point though, and that's that you don't need to fully introduce everyone in every story, and you don't have to start with an origin story. I know that Chris Claremont had a principle when writing the X-Men, which was that every comic is someone's first. That didn't mean "Continuity is useless" – anyone familiar with the Claremont era knows that it was an amazing soap opera of nested plot threads all over the place. What it means is that any comic needs to quickly introduce the main cast (often via a Danger Room sequence), and also remind the reader of any character that recurs. "Juggernaut! I thought we dropped you in the middle of the ocean!" — "Yeah, took me a month to walk back along the ocean floor." Of the MCU movies, I actually think the Incredible Hulk does this best – we see some newspaper clippings and such during the intro, but otherwise we're just dealing with a Bruce Banner that's already been on the run for quite a while. Guardians of the Galaxy sort of does the same, but it does form the origin story of the
group, if not its members.