I think there's a notion that the imposition of tarifs will ultimately affect the amount the country imports. However that stick only kinda-sorta works.
As a reference - in the country I live in, exports to the US rose in the year after "liberation day". Because of 2 things; Americans like to buy stuff and tarifs imposed on everyone doesn't cause importers to source elsewhere - it's just business as usual.
Take for example something as mundane as coffee. Do tarifs mean Americans drink less? (perhaps not). Do they grow more locally (well, no, not least because of ramp-up time to new production.) Tarifs are just a local tax on consumerism.
So take your example above. 25% of the Canadian GDP goes to the US. The question then becomes how much of that is going to be sourced elsewhere instead? And can Canada find an alternative market to make up the difference?
About 30% (of that 25% above) is oil & gas etc. That's easilly sellable on the global market. Not that they'll need to, the US customer will simply pay more. The next 21% is car parts, machinery etc. US Auto makers are already dependent on external parts - maybe they can get them from somewhere else? Somewhere without already higher tarifs?
Other big chuncks are Consumer, Forestry and Industrial goos (15%), Metas (12%) and agriculture (10%). None of these are easily sourced (cheaper) elsewhere.
In other words, tarifs make for good headlines. And there will inevitably be some winners and losers. But the effect on Canada's GDP will be minor.
I'm not even sure that Canada needs to reciprocate - Canadian consumers are very good at simply not buying American products. No govt can force the Canadian people to buy Budwizer....
>> If both countries decide to start ramping up tariffs, one of them is going to collapse and one won't even notice a difference.
I agree, but probably not the same way you think. It's US suppliers who depend on exports that very much will notice. American products are very expensive, and Canada is one of the few markets where they are viable. That's why DJT is so upset with alcohol bans in Canada. There are lots of markets for Canadian goods. And ultimately this whole process is encouraging Canadian producers to explore some of those markets, which in the long run is good for Canada.