As another canadian, here's something that occurred to me recently: we like to sling around the oligopoly argument when talking about telecoms, but when I bothered to look a bit into it, rather than finding some tightly knit mafia-like boys club, what I found is that the landscape is quite fragmented and messy (e.g. look at the scope of Telus' operations in Ontario vs Quebec, or look at how Sasktel operates, or look at companies like Fibrestream)
The big telecom lobbying argument vs CRTC about how urban markets need to subsidize rural infrastructure costs is not something 95% of canadians like to hear, but it kinda makes sense (They say rural infra simply isn't cost effective because Canada is so expansive, but you expect high speed Internet access in your Muskoka cottage, right?)
Banking is kind of in a similar boat in the sense that it's an industry with economies of scale effect, so naturally there are going to be big players. Even smaller players like Tangerine need to make "big boy" investments like call centers. ICBC is another example of a bank that isn't the big 5 and yet has brick and mortar branches to serve a highly specific niche.
OpenBanking doesn't mean that TD et al somehow get to tighten the noose on smaller banks to their own advantage; it's actually on them to implement the APIs. If Tangerine can't keep up with other banks improving their technology, that's their own fault. What the whole thing means is that Plaid doesn't get to have root access to your banking.