Well that is a tough question. I would say Jazz had it's most popular appeal during the swing era, (mid 1930's up through the end of WWII.
Jazz is the ultimate niche music, with so many styles and fusions of styles, that you really can't put it in a nice neat box.
Artists like Wynton Marsalis stay within a safe museum quality style, never branching to far out of the box, an artist like Marsalis tends to be at his best in a live setting.
some very good traditional artists that dare to experiment a bit are, Joe Lovano Vijay Iiyer and Jason Moran.
I have been listening recently to The Maria Schneider Orchestra and Jaga Jazzist, The former is a Gil Evans disciple that really has a unique style, Jaga Jazzist has been around about 10 years, they combine a melodic jazz style with modern electronic and rock music, not your typical fusion, but very original.
Jazz I believe will never die, as long as their are those within it willing to expand and grow the music, experimentation is jazz. I think it stays relevant drawing from many more popular styles and incorporating into the music.
jazz has been doing this since the 1920's.