The process of applying for and creating a "new extension" costs hundreds of thousands of dollars.
There have been past failed efforts to establish "new extensions" that required special browser plugins to make the system work. I suspect most people who bought into this system, by buying domains, were domain speculators who promptly lost interest and lost money in the process. Chances of such a system attracting new money are, at this time, unlikely.
Heaven only knows how ICANN's "unlimited TLDs" fare, but if you have a few extra hundred thousand dollars you can find out.
[edited by: Webwork at 3:50 pm (utc) on Dec. 1, 2008]