I agree with that, though if I'm TeeSpring, I'd fix it anyway. It's silly to throw out incoming business. I assume the same thing would happen if there were a brief GA outage.
Yes, though again, throwing out a checkout session for anyone running that extension isn't wise.
"If GA is unreachable then window.ga wouldn't be defined."
The legacy code you insert for GA does:
var ga = document.createElement('script'); first, then loads GA. Wouldn't that have the same issue if GA is unreachable?
The author is running an extension that specifically injects code to make window.ga into something unexpected, and then complaining about it.