I doubt it adds much to the discussion of whether 15 years is an appropriate time to develop a worthy sequel, but Wikipedia's Incredibles article has these quotes from Brad Bird, who created the story and directed the movie (also playing the role of Edna):
"Director Brad Bird stated in 2007 that he was open to the idea of a sequel, if he could come up with an idea superior to the original film: 'I have pieces that I think are good, but I don't have them all together.'
During an interview in May 2013, Bird reiterated his interest in making a sequel: 'I have been thinking about it. People think that I have not been, but I have. Because I love those characters and love that world.' He continues, 'I am stroking my chin and scratching my head. I have many, many elements that I think would work really well in another Incredibles film, and if I can get 'em to click all together, I would probably wanna do that.'"
His thoughts seem to be very similar to Chronicler's, in that he wants to conceive an idea that is superior to the first movie's (a titanic feat in itself), and then wants to incorporate all of the lesser pieces that he has in his mind, without making it seem rushed or cliched. He's also stated that he wants the sequel to shy away from the superhero movie trends that emerged after the first movie was released (I assume he's referring to the Marvel and DC philosophy of expanded and interconnected universes), and wants to keep the focus on family dynamics. All of these expectations are undoubtedly difficult to combine, especially given audiences' expectations for superhero films in recent years, and Bird has been involved in other projects that likely have taken time away from the sequel.
Yes, 15 years is a long time, but I think it's acceptable, given the circumstances and the expectations of the fans.