Conventions are not a one off thing. The memorial cup for Kingston would be.
Generally speaking, the only ones booking somewhat last minute are the fans of the teams that win their league championships. Each of those teams get 200 tickets and usually release around 60-100 of their tickets to fans for purchase. So, maybe 100 “last minute” rooms.
Kingston will have around 5700 seats. 4700 for sales. They will likely sell around 4000 or so to locals with about 700 to the people that attend most every year or people speculating on their team winning and they want to have tickets in hand if/when they do.
Access to hotel rooms for those coming in last minute isn’t a big deal. There are dozens of motels on Princess st. all relatively cheap. So even if the prime downtown hotels are fully booked a week before the event, there are loads of other less desirable options but options nonetheless. This doesn’t include Gananoque which would be a great spot to stay if you are more interested in tourist activities during the day.
There are also a multitude of campgrounds and cottage rentals in the area. Loads of lakefront cottages for families within 30 minutes of downtown.
Affordability should not even be in the discussion. If you have $700 to drop on a Memorial Cup ticket package then you have the coin to drop on whatever the other costs are. I have zero sympathy for those spending $100 per game on a junior hockey ticket that complains about $150-$300 per night for a hotel (depending on the class of the hotel). That pricing is in line with most tourist type towns including Niagara Falls where most hotels during this years Memorial Cup schedule fall on line with the $200-$250 per room price tag per night.