Actually, I think I recall there being more than two marathons that shut down the whole downtown core surface transit.
The Santa parade actually allows the streetcars to get through. Parades have gaps that allow for this. Caribana doesn't affect any streetcars. I don't remember if Pride parade allows streetcars through, but it does not affect all east-west streetcar routes downtown as the marathons do.
Marathons do not have gaps and you can't just stop runners for a few minutes periodically to let streetcars through the way you can with a parade. For whatever reason, they make sure the marathon routes cut off every single east-west surface transit route in the entire downtown core. That's simply absurd. This is why "alternate routes" don't work (unless you think a 10-mile diversion is your idea of practical). And almost nobody is going to have prior knowledge that the marathon is happening...it simply isn't common knowledge.
Of course we should all the events we want...including all the marathons people want to organize.
The serious issue here is shutting down public transit. You simply do not do this. And Toronto is famous for screwing up transit (especially streetcars) willy-nilly....they are just so unbelievably cavalier about it too. The idea that it is ok to shut down public transit for anything less than an unavoidable disaster or emergency is very, very stupid. It isn't called an essential service for nothing.
The crazy thing is that the solution is just so easy.... coming up with a suitable marathon route that doesn't interfere with public transit....or at the very least not paralyze the entire downtown core's surface public transit.