Winter in Montreal just needs the right clothing, but then you’re not packing light. A proper winter coat, gloves, boots, etc. will need way more space than exists in that tiny backpack.
I've spent a few weeks in Tokyo and Kyoto mid summer (though less extreme I guess than this summer) because of business trips and you can "manage"--in the sense that you can limit activities during the hottest parts of the day or just in general--but I don't really recommend picking visits at those times unless you have some specific reason to.
Montreal in winter on the other hand is fine. But, as you say, you need the right clothing which can be reasonably compact but isn't going to fit in tiny luggage. (And activities will be at least somewhat different from in summer.)
I'm a fairly compact traveler in general (just carry-on usually) and you can get off with a lot in temperate to warm climates in urban locations where you don't need to dress up. But I also don't like to pack only those things that I'm sure I need. For example, I have a little kit bag of miscellaneous stuff I mostly don't need but am sometimes glad I have.
Depending on where you live it might be winter already there too so not necessarily a problem as you already wear the shit and remove what you don't need during the flight. Otherwise you just have to wear them when you enter the plane and put them back in the ikea bag that was in the coat's pocket the rest of the time.
Tell me you’ve never lived in a place with actual winter without telling me. The clothes aimed at locals are designed to be a 5 to 10 year investment. Not a jacket you buy to throw away in spring.
That said, you are right that unless you’re in a place like that, the correct gear isn’t even available for you to buy.
But while you’re shopping, and the trip in from the airport would be pretty miserable.
The "cheap, serviceable, but used" is... fairly picked over by the locals (and especially the homeless).
Consider San Francisco for a moment and the daily occasion of tourists who suddenly find that the fog rolling in the evening is cold and then buy the "cheap" (but horribly marked up) jackets that aren't the right size in which they stand out like sore thumbs.
Or they could go to the Salvation Army store and buy a winter jacket - consider the availability of them there.
Or they could go to REI or Farm and Fleet (I know there aren't any F&F in SF) and buy a new winter jacket there.
After childhood, locals have winter clothes that often last 5-10 years or more (and they pay for durability). My winter jackets are from '10 (I've got a medium weight photographer's jacket that I got in Keeble & Shuchat in '01 that I still wear... wish I could find something like it again). My father's winter jackets are in the 10-20 year range.
I'm also going to note the plural. Locals will often have two or three "winter" jackets depending on the weather.
The only time you get a jacket for a season that is cheap is for a child who is going to outgrow it by the next winter.
If I buy winter clothes in my home country, the clothes will be ugly, expensive, and I have no way to make sure if they even work for their intended purpose. It's too hot here.
But when I go to countries that have actual winters, especially First World countries, then I can find decent winter clothes even in, say, a Target.
> The clothes aimed at locals are designed to be a 5 to 10 year investment
Not if you shop at Target :)
We're comparing https://www.target.com/p/cutter-buck-mission-ridge-repreve-e... (and honestly, the puffy jackets are... to me, as a local to the upper midwest, those... they're not what you see many locals wearing... some, yes... but not many) to things in https://www.rei.com/c/snow-jackets?ir=category%3Asnow-jacket...
Easy yes, cheap, not so much.
We had similar in AR back in 21 and I was outside in a tshirt or if more than 10 minutes the thinnest windbreaker ever. (Single layer of plastic and compresses to a baseball size or so.) I might go to a hoody at -20 but not sure even then.