Krull is fine. It's the fantasy genre that is ridiculous.
This film gets a lot of things right about fantasy films. The acting is very good. The special effects are respectable for the time (Star Wars was game-changing state of the art). The creatures are interesting. The villain is formidable, and he has some very interesting abilities. The set and costume design are mostly good. There are some genuinely scary scenes in this film.
People generalły complain about the flying horses, the love subplot, the flying weapon and so-on. You can change the flying horses to giant dragons with wings, change the love subplot to a royal prophecy, and change the glaive into a magic sword. Change the Alien King into a demonic spirit. Once you do so, then you essentially have Lord of the Rings which is the prototype of all modern fantasy novels and films. The beast really isn't anymore ridiculous than Sauron and the Witch King in either theory or appearance. The flying horses aren't anymore ridiculous than flying dragons or even 20m elephants. I would add that the alien army of Krull is somewhat less ridiculous than the ghost army of Return of the King. Aliens almost certainly exist somewhere. Ghosts almost certainly do not.
Sorry, but the acting in the LOTR trilogy was no better than that in Krull. Several actors in the latter film would go on to do much better things.
Worse, I believe that the artwork of Peter Jackson's movies borrows liberally from this movie. Orthanc, Baradur, Minas Morgul, and many other sites generalły resemble the interior of the black fortress. The appearance of Sauron and the Nazgul armor generalły resembles that of the beast's soldiers. Being a b-movie maker, Jackson almost had to have seen this film when he was young. I don't doubt that he was strongly influenced by the artistry of Krull. Considering this, one could hardly hold it against Krull for borrowing from Tolkein's books.
Just about anything that you could say about this movie, you could also say about Lord of the Rings in some way. You could also say it about Dragonslayer, Reign of Fire, all versions of Conan the Barbarian, the Hobbit and LOTR, MCU's Thor films, Masters of the Universe, The 300 series, etc. You might even make similar complaints about John Boorman's Arthurian epic Excalibur, which I and many paid critics see as a classic. None of those films are any more plausible than Krull.
There never were any fire-breathing dragons. There never was magic. Prophecies are just words no more potent than what you read here. Goblins and Trolls never existed. To say that one is any less ridiculous than the other is inherently illogical. You either like fantasy material or you don't. It is all inherently ridiculous. They all suck to some extent. You just embrace that ridiculousness and run with it. Krull is fine. The people who don't like it don't like the genre.