I don't disagree that Bombardier commercial aircraft was struggling before that tariff - but it absolutely was the nail in the coffin. They actually ended up selling off the C-Series to Airbus for 1 CAD as a symbolic f-you to Boeing over the whole affair. The WTO ended up finding in Bombardier's favor but it came in too late to save the company from the financial damage.

Whether the C-Series would have been profitable for Bombardier without the extreme heavy-handed US intervention is not particularly clear but the outlook already wasn't great - still, it's inarguable that Boeing leveraged lobbying to quicken Bombardier's demise.