"Except that it does keep the oil from being burnt long term. If it increases the cost of extracting and refining oil, then the point where it becomes more economically feasible to use alternative sources is pushed forward."
But that's not what's happening. For one, the pipeline expansion has no effect on the price of extraction or refinement. Oil is a commodity, so its price is set globally. It's not as if requiring Canada to only uses the already extensive existing pipelines between the oil sands and the Gulf refineries will make oil prices skyrocket. It only means that Canada will sell crude to a lot of countries rather than primarily to America.
Also, increasing the price of oil may be exactly the opposite way to make alternative energy sources viable. Higher oil prices mean companies will invest more to extract oil which isn't worth getting now. That's what's already happened with the tar sands and what spurred fracking to begin with (although now that the technology is widespread the barrier of entry is lowers).
Low oil prices, on the other hand, force energy companies to diversify. When prices are low they make less money, so to make up the slack they would invest in developing new technologies (even more so than they already do).
Also, let's keep in mind that, if man-made global warming exists, it's impossible to stop. China and India, and other developing states, will not hold themselves back industrially to the extent needed to avert the predicted disasters.