While I cannot look forward 3 years in time, with today's knowledge when the Central subscription expires, the Instant clusters will return to local management. The clusters will continue to function.
You can decide to extend or renew your Central subscription, or use Airwave or just the local management, or even add a controller and make the deployment controller based. So many options.
Though in the case you use Central cloud services like Cloud Guest or Presence Analytics, those features will stop.