Question

What happens when v6 comes out? Are we forced to upgrade if using Sphere or are we able to use v5 as long as we want?

Answer 1

Sphere does not force you to upgrade.  You can stay with 5 forever if that's what you want; however, once 6 comes out, there will be no more updates to 5, and you will not be able to access any add-ons that may be developed exclusively for 6 or higher.