Unless there will be new separate Windows OS created that is not backwards compatible with anything prior to it like it was attempted with Windows S, this most likely will never happen.
Local accounts are integral part of OS. MS might make it harder to do, but there will always be an option.
Probably not, all you need account for is for sign in after all. MS account just has additional benefits related to syncing your settings and some settings enabled by default like it is with this OneDrive feature and BitLocker encryption, but most of it can be replicated afterwards with local account.