There's 8-bit MCU's all over the place. I tried to find some numbers for you with a quick Google. This 2014 Amtel report puts 8-bitters at $6+ billion a year. Even the 4-bitters are still making hundreds of millions a year. I include a bonus link on that if it perplexes you.

http://www.atmel.com/Images/45107A-Choosing-a-MCU-Fredriksen...

http://www.embeddedinsights.com/channels/2010/12/10/consider...

Far as future, Jack Ganssle of The Embedded Muse has a nice assessment of it summarizing various sides:

http://www.ganssle.com/rants/is8bitsdying.htm