If nobody buys anything, Canon will be forced to release the 5d3 sooner. If people keep buying the current 5d2, the next model will never come out. Simple economics.
No. It's not.
It not as though Canon or any other manufacturer has a bunch of new models sitting around on the shelves just waiting to be released. New models are released when the technology, tooling, testing, manufacturing lines, etc. are ready. A drop off in sales may prompt a company to review whether or not they need to refresh the product (or drop it altogether) but it's not going to have much impact on a specific release date.
Even if that theory were valid, the total number of people who even visit this site represent a tiny fraction of the market. We can, in no way, affect that overall market beyond a fraction of a fraction of a fraction.
Canon is a business. Their purpose is to make money. They do that by selling products that people want to buy. They would purposely hold back a product that there is demand for because...? They don't want to increase their profits and sell more products? They don't want to satisfy customer demand and foster brand loyalty? They just want to be mean to you?
Doesn't it ever occur to some people that they don't want to release a product prematurely because they have a reputation to protect and a premature release of a product before it's fully developed and tested would damage that reputation.
You can hold your breath until you pass out, but it's not going to affect Canon's release date in the slightest.