Here's a scenario that plays out in WoW all the time and it happened to me. Basically, a user quits playing WoW and their account gets hacked at some point after they quit. The hacker then turns on 2 factor auth via the WoW authenticator app. It is now impossible for the original user to log in to the account or reset passwords. To fix this you must argue and explain with customer support that the account was hacked an that the 2 factor auth is preventing you from resetting passwords and such.
So, unless you turn on two factor auth up front for all users, it's going to actually make it worse for the end user if their account gets hacked. So, like captchas, it's solving one problem and creating another for the user. I'm not sure that is the best solution.