It's not so much that they'll leave, as much as some percentage will abandon during the signup flow. I know somewhere out there are statistics on those who have to click a link in an email only to get distracted by other emails, to say nothing of the time to fill out forms, create a password, save to password manager, open your 2FA app for the more advanced users, etc.

The higher the friction, the lower the probability of conversion. E.g. Amazon famously found every 100ms of latency costs them 1% in sales.

At its most simplified, this can be thought of as a simple function of time — the more time something requires, the higher chance something else happens during that time, invalidating the original task.

The best sign-in flow is none at all — that's what e.g. Discord does. They let you use the app immediately, with an automatically created provisional account. Amazing user experience.

This applies universally — convenience is everything.