React 16 adds Portals and Error Boundaries, which are very useful things that often change the way you write components. When you defer upgrading something that

React 16 adds Portals and Error Boundaries, which are very useful things that often change the way you write components. When you defer upgrading something that changes the way you write code, you accrue debt because you're increasing the amount of code written in a style that will be deprecated once you upgrade. If you're all-in on React (as Discord is), the upgrade to React 16 is inevitable. Given that, upgrading now makes sense.