Only horse racing and boxing have declined in absolute terms. No team sport has ever really declined. Some have stalled, like MLB or the CFL, but the sports pie just keeps growing overall.
The interesting question is, what happens when the sports market itself starts declining someday? I'd guess, right now, that the CFL has a higher likelihood of surviving that than MLS does. Just based on habit and history. A lot of MLS teams have highly leveraged owners (including TFC/MLSE) who can't really sustain big downturns.
The Argos are a weird special case. It's a seriously tarnished brand in this market, very analagous to TFC in 2012. They need a Giovinco.