Re: AGILE means "let the users test for you"
I agree most companies have a shitty understanding of what "Agile" really is, and use it as an excuse to cut QA and therefore costs, but, for things like this, at some point you can't test at sufficent real-world scale, other than releasing an opt-in tech preview and seeing if/where it breaks.
Even that doesn't scale up to the true load, but it's likely more than can be achieved with in-house testing.