I certainly agree with you that if they did that, it would come dangerously close to a bait-and-switch and open them to massive lawsuits/class-actions.
Now, I'm a photographer/musician, so when it comes to legal contracts and licenses and such, I am not within my area of expertise; my eyes glaze over and I respond with a hearty "Pfooey!". But... and I'll admit I didn't look into this yet, don't most licenses have a clause in them stating something to the effect of "We reserve the right to revoke this license at any time and basically for any reason, so sod off!"? Wouldn't that be their way out of any liability?
At any rate, I emailed Serif/Canva support and asked specifically what assurance I have that I will be able to continue to activate and use my perpetual license "in perpetuity" in the event that I, for example, upgrade computers and need to reinstall. And specifically what will be my options when that authentication service inevitably disappears over the long term. I shall report back my findings to this thread.