Check this out.

https://twitter.com/johnhenderson/status/858864571347611649

Please elaborate ... what's the "power of incumbency" in this context?

I know what it is for politics, but I'm guessing here.

Right, I'm not sure what that word means as used in this topic.  Maybe (semi) permanence vs. impermanence?

I think it refers to the power of an addicted userbase? Meaning the users are already "there" (i.e. "in office" i.e. Instagram users) and they would rather be using IG, but IG doesn't have stories. Oh wait, it does, and I'm here, so I stay here.

Instagram: 700m active users (aka "incumbents"?)
https://www.statista.com/statistics/253577/number-of-monthly-active-instagram-users/

Snapchat: 300m active users (aka "incumbents"?)

Service with 2.33x as many "incumbents" launches new service and competitor with similar service immediately surpassed.

The Instagram vs. Snapchat War: 43% Say They Would Delete Snapchat If Instagram 'Stories' Adds Filters
http://www.huffingtonpost.com/wishbone-app/the-instagram-vs-snapchat_b_11555342.html

Good enough and convenient. That's proved a winning strategy for Instagram's clone
https://techcrunch.com/2017/01/30/attack-of-the-clone/