Pierre Trudeau was written off for each of his last three elections. He ended up in power from 1968 until 1984, broken only by the 7 months of minority PC government under Clark.
The Liberals aren't campaigning, while PP has been burning cash in election mode for months. Once the writ is dropped, it's a completely different ball game. The Liberals were in third place behind the CPC and NDP when the writ was dropped in 2015. Campaigns matter.