Can’t say I agree. Julie Louis-Dreyfus has had the most post-Seinfeld success, but all of them have had success to various degrees.
For one, Jerry Seinfeld went into the show as a stand-up comedian who had been given a sitcom. He was not an actor by profession, and has never shown an inclination to be one. He still did stand up before, during, and after his run on the show. He was making millions of dollars a year doing stand-up for at least 15 years after Seinfeld went off the air. I’m not sure how often he still tours, but he was by no means sitting at home doing nothing. He also created the short interview series Comedians in Cars Getting Coffee, it ran for like 6 years on the small-time streaming network Crackle, but then got picked up by Netflix and I believe is still going on.
Jason Alexander probably had the strongest acting resume going into Seinfeld and had a background in stage (he won a Tony in 1989), and while he hasn’t been as successful as JLD, he’s kept busy as well. He went back to doing stage acting somewhat regularly after Seinfeld, and had his own TV series albeit only for one season, he’s appeared in smaller roles throughout too. I think it’s hard to qualify him as a one-hit guy. It also kind of ignores he was already kind of an established actor going into Seinfeld–he had his role in Pretty Woman, he also had a decently big role in the Jack Black vehicle Shallow Hal. He did voice work in several animated movies, and he also was the voice of Duckman which is a show that has kind of been forgotten to time, but it was one of the better known “adult” cartoons of the 1990s and ran for a number of years.
Michael Richards is probably the closest on the cast to a one hit wonder. He actually had some comedic acting roles under his belt that occurred either before Seinfeld or coterminous with it, he was the bad guy in the John Ritter vehicle Problem Child in 1990, and had a few smaller roles in wide release comedy films that came out during the same time he was on Seinfeld. He also had probably 20+ acting credits for random guest appearances or small character roles on various TV shows. However Richards is somewhat of a special case, in 2006 he went on stage at the Laugh Factory doing a stand up bit and got heckled, and infamously responded by screaming racial slurs at the hecklers for 45 seconds straight. That mostly ended his career, so it’s hard to say if he’d have done anything after that.
I’d object to Don Knotts on a few grounds. For one, he never had what I’d identify as a huge break out role where people expecting “more” from him. He had been steadily working for about 9 years before he got cast as Barney Fife, and his run on Andy Griffith was “a career” by any measure because of how much money and residuals and etc you get for being the #2 star of such a major TV show that ran on network television for so long. Remember being on network TV was a huge deal in the 1960s when there was only 3 networks, and Don was on network TV for 8 years and 162 episodes of that show.
But after Andy Griffith he largely continued to get similar levels of work–never the star, but often a comedic foil. Another big run he had was over 100 episodes of Three’s Company as Mr. Furley (when the role got recast), which was one of the major comedies of the late 1970s/early 1980s. Two very big TV runs on network TV, plus a good 70+ credits doing character actor work, just hard to see that as anything like what OP was talking about.