Will the New Launcher Cause Delay for Custom Launchers?

Now do not get me wrong, the Vanilla launcher is nice and all, especially the new one going official in 1.6. But for some people, this is not enough. There are other nice launchers out there that add features like multiple saved accounts, multiple versions and modpacks (the new launcher supports this to an extent), and other shinies.

According to a tweet by Dinnerbone: "New launcher just went out which enables Yggdrasil, our new auth system. So much more secure than our old one, and much more featureful!" The new launcher has a different authentication system, meaning the way the MC login servers verify your password and username has changed. Because the old launchers will still work with versions 1.5 and below, I assume the old auth system will still be in place to some extent. But will it be supported for 1.6 and beyond, or will custom launchers be required to update to the new auth system? Most likely they will need to keep the old auth system in place for versions 1.5 and below while for 1.6 they will be required to use the new system, probably causing a little delay for those custom launchers that only have periodic updates, and deprecating those launchers that do not update anymore.