Actually, the G&M published an article some time ago giving part of the rationale (https://beta.theglobeandmail.com/news/national/diagnosing-canadian-doctors-anger-over-trudeau-liberals-tax-plan/article36442800/). Line 3 is likely to provide sufficient pipeline capacity for bitumen until 2030. By then, demand for oil may well be diminishing rapidly given both the rise of electric cars and the GHG reductions demanded by the Paris accords.