United has an incredible app and great user-facing tools that I can only imagine took a large investment to build out the tech across so many pieces of their infrastructure: at the gate, at baggage, at flight ops, etc.
If they introduce 2FA, the bar is set so high that I'm fully expecting super advanced tools (that really aren't that hard to implement) like time-based keys, hardware key support, passphrase support (not really 2FA tho) atop the usual SMS/email offerings (which are still pretty insecure)