How do you calculate those numbers though?

It's not like your colleagues will be keeping track of how much time they've wasted writing ineffective code. If anything, they'll try to hide that by arbitrarily inflating sprint points etc.

I've worked in environments like that and the issue almost always isn't that people wouldn't LIKE it if there were tests, it's that they

1. Don't want to have to learn something new in order to do the same job they're already comfortable with
2. Are worried that if they convince management to let them invest X amount of time into doing something that will improve productivity, they'll be expected to be more productive in the future

And of course, all of this for no extra money. Unless you work at a place where management prioritzes developer happiness over how many sprint points the team can knock out every week (and those are rare), the sad truth is that it'll likely be about as popular as leftover food growing mold in the community fridge.