GT's are still around and going strong - especially in the racing scene. I know they sold off Dyno a while back and Toys r us had the brand for a while. I don't remember how long GT kept Robinson around, but I know they are no more. I still remember my old GT mach one from back in the day. I actually owned that bike for 21 years, and finally gave it to a student after fully rebuilding it. Some of those older frames that weren't considered "cool" back then actually have the same specs and geometry of today's "cool" frames. The cool ones from back in the day, like the Specialized Fat Boy, they look ridiculous by today's standards.
But most of the big brands are still around: Haro, Hoffman, GT, Redline. Diamondback just ditched their team riders, so I don't know if that's pointing toward things to come for the company as a whole or not. Check out bmxmuseum.com for a trip down memory lane.