The thing that kept the FF relevant for so long was this idea that they were just in a completely different class than anyone else in the Marvel Universe. They’d been at it longer, they handled bigger threats, and they had concerns that went well beyond other heroes’ struggles with secret identities and making sure Aunt May could get her prescriptions. The Avengers had some great characters, but they weren’t the team you were going to call when Galactus showed up or if you needed to dive into the time stream, you know? Captain America might save the world, but the FF were out here saving the multiverse.
There’s actually a great story about that same idea. It happens in Fantastic Four #334, during Marvel’s greatest crossover, Acts of Vengeance. The basic idea is that due to some shenanigans from Loki, all of Marvel’s villains decided to switch opponents to see if they’d have better luck dealing with each others’ arch-nemeses. It led to some great, bizarre match-ups, like the Punisher fighting Dr. Doom, Ultron taking on Daredevil, and that great issue where Magneto shows up in the middle of an issue of Captain America to drop Red Skull into a hole in the ground to die slowly for being a Nazi.
The Fantastic Four, on the other hand, got to deal with a bunch of B-list Spider-Man villains like the Beetle and the Shocker, and wound up dispatching them pretty effortlessly. They even clown on the very idea that these guys could provide a threat on the covers, and it makes perfect sense. The FF dealt with a giant purple space god coming down to eat the planet, are Plant Man and some dude with pocupine powers really going to provide a threat?
It’s worth noting, by the way, that this issue comes in the same run by the legendary Walt Simonson that also includes the tongue-in-cheek introduction of the “New Fantastic Four.” It’s a great bit of commentary on changing tastes, slapping together a very commercially viable team of Spider-Man, Hulk, Ghost Rider and Wolverine for a mission to rescue the real FF. Even then, in 1990, Simonson and artist Art Adams were asking if the FF were still relevant.