The argument that Canon is "sitting" on anything is pretty bad, but ironically it's just that one poorly chosen word that ends up doing a lot of dirty work. Many of the underlying assumptions that argument makes are right, but it spins them and assigns something sinister to the situation.
The 5D Mark II does what most people who buy it want it to do: not sports or wildlife photography, but portraits, landscape, and as a low-cost body for motion pictures using EF lenses. Nobody has said that it fails as a competent camera - everything that people are looking forward to in the Mark III (or any new camera) are things that would be nice but with which people currently are doing without. The next camera may well be more expensive, after all, and when you count inflation against even a steady price, and falling prices on secondhand units, the current model actually gets cheaper to buy. As with the film body days - if Canon can sell a perfectly good camera year after year, they save money and will be happy to do so.
This also helps the average photographer too - what a lot of the "Mark III sitter" crowd seem to forget is that when a new model comes out it adds pressure on current Canon users to spend their money upgrading to a model that they really don't want or need, but sometimes may feel they have to just to stay competitive with other photographers.
There is still a big, big difference in the implied intention between selling a good camera for a long while - even past its prime - and the arguably libelous and unflattering accusation that Canon is "sitting" on good technology just to milk profits. The argument conjures up images of "fat cat execs" swimming through piles of cash and unreleased cameras in a vault like Scrooge McDuck. Canon, and every other manufacturer for that matter, always has access to newer, better technology than what is out on the market at any time, but developing that into products is not like waving a wand. They are under pressure from competitors and while the Mark II's spec is dated, the last thing they want is to release a camera that will not be a good purchase (an investment, really). The unfolding changes with video on DSLRs is only making things more complicated.
Personally, I'm excited about the evolution to video but I also hope that Canon doesn't lose sight of the importance of continuing to tweak the interface for stills photography and move away from some of the thinking from the film days. It is a pain for many serious photographers that the new cameras have dedicated buttons related to moviemaking, but there is still no apparent commitment to making Mirror Lock-Up a dedicated button. Auto ISO is another feature that needs more development, especially since ISO adjustments only make sense up to a certain point due to the response curves of sensors. The possibility of mixing the video and photography development groups with the aim of "one-size fits all" systems is an exciting possibility, but it also will be challenging.