Putting things is perspective, you can take opera as an example. There hasn't been a great opera produced since perhaps Puccini's Turandot in 1926; but opera did not reach its peak until Pavarotti and the Three Tenors 50-60 years later. There are probably more operas being produced today than any other time in history.
Jazz has actually faired better in the sense that there are many people creating quality jazz now. And there is definitetly more jazz going on today than any other time in history, and on a global level.
Historically, music had never seen the type of mass popularity associated with The post-Beatles world. 'Bird' never played Shea, at least not to my knowledge. Well, it wasn't built yet. He never played Yankee Stadium. I think they asked Monk but he couldn't work it out with their playing schedule.
Great jazz was always an underground movement and people who want great music are still turning to jazz. Then there's Brittany and Pink, those lil hotties. Well, you know I hear all the time about how the great days of AC/DC and Aerosmith are gone and Brittany and pink are here. Rock Band is bringing that great era back. Yeah, Brittany sucks but AC/DC is great. It all gets absurd for me, esp when these guys are still parading around like school boys at 55, or the Stones are still complaining about not getting recognition after they have dominated the music industry for 40 years. But you can listen to a great jazz ensemble like Dave Hollands' as Stan mentioned, and know that there is some great music going on there.