Leave a Drupal project idle and unmaintained for a year, and a new, non-backwards-compatible Druoal version has been released and you'll need to rewrite everything.
I don't have experience with Drupal, but it says "plan for periodic upgrades of their project to the latest major release (every 6 years or so)". So, even if a new release comes out, you have _literally several years_ to decide when to make a switch. For perspective, Backbone.js was released 5 years ago!
But the "old" major version will still be supported for a very long time frame. In the case of D6, it was supported over five years after the release of D7.