The Fantastic Four aren't the only characters created in the 60's, some were even created before then (Namor, Captain America, Jim Hammond). Yet they are the only ones who are usually called "dated" or "boring". Why is that?
I personally think the main problem is that writers are so beholden to the Lee/Kirby run that they don't don't do what the FF are best at. Innovating. This leads to the same stories and villains cropping up again and again in different forms. It also leads to my point: Reed Richards is often the only important character.
Reed is the sole driving force for any endeavor the team goes on. When was the last time, if ever, have the others said lets go here or I want to investigate this? It's incredibly rare and because he is the focus of the adventure he is usually the focus of the story, there are several FF villains that consider Reed and only him their rival and the others are just ancillary. There barely any villains that consider Ben, Sue or Johnny their personal enemy. When you think about it, if they didn't have their powers what would be the point of them as a team? Reed would still be the leader, scientist, inventor and explorer. But the others:
Ben - Would be Reeds muscle, a job that centers around Reed and could be done by anyone. What would be Ben's motivation without Reed or his powers? What does he want to do? If his motivations surround Reed then he is nothing more than a support character. His piloting skills, the one motivation that is all about Ben, barely come up nowadays.
Johnny - Has no reason to be on the team as he would provide nothing. There was potential with him being a mechanic, but that is eclipsed by Reed's inventing and fixing everything (which shouldn't be Reed's field... but comics I guess) which makes Johnny superfluous.
Sue - Without her powers she has the least reason to be on the team. She provides nothing. Her role is the same as it's been since her Invisible Girl days: Reed's girlfriend turned wife.
The Fantastic Four isn't an equal team and never has been. You can tell by how Reed treats the others as he hides things from them constantly that he doesn't consider them his equals and honestly, they aren't. Reed has the most character arcs and chances to grow which is one of the most important aspects if stories - character growth. They are: being more humble, learning to value his family over work, learning that he can't solve everything (especially alone) and to stop hiding things from the others. These themes are repeated over different runs, but they are still more than the others. Ben's was to accept him being the Thing, Johnny's was to grow up and Sue was to be more confident in herself (with Malice sometimes thrown in for good measure). Ben and Sue have already overcame these and are now left with nothing left to grow from, while Johnny is still in cycle of growing up then reverting with the next writer.
This needs to change. This isn't a team, it's a glorified Reed solo and it doesn't work in this day and age anymore. The runs considered the best outside of the original Lee/Kirby (Byrne and Hickman) either took the focus off Reed and/or made the book tackle grand concepts again that weren't a repeat of the original run. I love the Lee/Kirby run and what it did for comics, but that run was definitely a product of it's time and things need to change with characters. It's called the Fantastic Four, yet a lot of the time it feels like it's just Mr. fantastic.
TLDR - I think writers shouldn't be so cautious to change things because of Lee/Kirby's legacy. Also, the focus needs to be taken off Reed and distributed equally among members.