We’re told that Canon is working on a new EF 70-200 lens, though the source didn’t know if it was going to be an f/2.8 or f/4 lens. The EF 70-200 f/2.8L IS II was announced in 2010, while the EF 70-200 f/4L IS was announced in 2006. Going by that, the f/4 version is the likelier of the two to be replaced next.
The last EF 70-200 f/2.8L IS replacement only took 9 years, so it’s possible they’re already working on its replacement. It’s definitely a cash cow you’d like to continue to upgrade.
Some have noted that the EF 70-200 f/4L IS has disappeared from the Canon USA web site, but that is a fairly common occurrence for whatever reason and has never signalled an imminent lens replacement as far as we can remember.