>There is some foreign competition via roaming, e.g. both T-Mobile and AT&T have US mobile postpaid plans with unlimited Canadian roaming, https://www.att.com/international/canada-roaming/ & https://www.t-mobile.com/cell-phone-plans/use-your-phone-in-...

Those don't seem that much cheaper compared to local competitors. For instance the cheapest plan at&t offers is $30 USD/month, but public mobile is offering a 40GB unlimited talk text plan for $29 CAD/month ($21 USD).