Every update and upgrade to macOS requires you to specifically begin the process. That being said, once High Sierra is released, you can choose to not perform the upgrade and stick with Sierra. The current version, as you already know, is 10.12.6 and Apple might continue some security fixes for Sierra, but it will shift its attention to High Sierra. So there might not be that many more updates to Sierra once High Sierra is released.
Most applications have moved to Yosemite 10.10 as the base for operating, so you'll be set for a couple of macOS releases before apps move to either Sierra or High Sierra as the base version of the OS at which point you will have to either upgrade or risk running the older version of an application..
At some point, you'll likely want to upgrade to the newer version of Logic which will work for a few more future OS'..