Most of the auth platforms I've used allow ways for you to migrate users off of their platform. You'd want to make sure at minimum you have their email address

Most of the auth platforms I've used allow ways for you to migrate users off of their platform. You'd want to make sure at minimum you have their email address, but worse case scenario you'd require all users do a password reset essentially when you switch to the new auth provider. That's a horrible experience--but it's still an option. Better options actually allow you to migrate the user over as they login. You'd need to keep the "old" auth service around for a while though until you close the door on migrating. There's also an argument to be made that companies like this could (certainly not guaranteed) employ security experts and have better implementations (through libraries) as a result. Those implementations could (if updated appropriately), yield a more secure auth solution. In that regard, going with a third party, who has expertise, could be "better". Also, unless you're going to host your database that stores user data on prem, you're likely trusting a cloud with it. If I missed the point of your question I'm sorry. Did my best to answer. I have no relation to any of these services, other than I've used some at times throughout my career.