They haven't announced heavy restrictions on access to calendars. Anyone can still write a client to access Google Calendars. They just need to use Google's calendar api instead of caldav. The method of access has changed but not the level of access.
Also - I doubt the FTC has any power to force a company to provide email services - it's not like they're a phone company providing the only service available to a specific area.
Besides - they don't have to "shut down" Gmail for it to become undesirable to use it. They've announced heavy restrictions on access to calendars - how long until IMAP access is discontinued and they force people to use webmail or one of their apps?