While it's true that the shark jumping episode had wide viewership, and the show continued to draw in enough viewers to avoid cancellation until 1984, Happy Days is the impetus for the creation of the "jumping the shark" phrase for more than just the obvious reason. If you ever saw the first season, it was an earnest show about coming of age in the 50's, shot on single camera and taking itself fairly seriously. It was tonally very similar to The Andy Griffith Show or even Leave it to Beaver. That season got pretty good ratings, but in season two the show plummeted. So Garry Marshall sold his soul and turned it into a gimmicky nonsense show shot in multicam. The shark jumping episode was in season three when they were still desperate for a stunt to get some press and draw in more viewers. The phrase "jumping the shark" still very much means the moment when a show gets desperate and tries some kind of stunt (whether literally, through casting, or otherwise) in order to get a bump in viewers for a once popular show. It absolutely does not describe the moment a series peaks, and it never did. The guy who coined the phrase, Jon Hein, intended it to mean exactly what most people think it means.
Hein first used the phrase in 1985, not long after Happy Days ended, and was pointing to the moment as exactly what it was, a pathetic ploy to appeal to the lowest common denominator (the entire episode is about the gang going to California - travel episodes are one of the oldest gimmicks in the book) and increase ratings. After season one, you could argue that entire show was jumping the shark every week, even before they literally did so. They employed gimmicks, bad jokes, and writing so dumb a four year old could follow along for a reason. It worked. The show didn't remotely peak in season 3, not in ratings and certainly not in quality.
Shows like Seinfeld never jumped the shark because they never had to, or never chose to, pull such stunts to chase ratings. Seinfeld did take a season to find its voice and pacing, but once it locked in it never compromised.
I don't know what to say about the Marvel stuff. Hard to jump the shark when that's your baseline.