That’s fair. When I think of zero-sum, I think of it in terms of his industry. i.e. To use Basecamp, you’re stopping the use of another project management tool. So, there’s a set number of customers seeking those tools and they’re either using BC or a direct competitor.
So, my point of it not strictly being zero-sum, is that there are people, for example, who use Gmail and Google Sheets to manage projects. If someone switches to BC from using Gmail/Sheets that’s no longer zero-sum because those Google products (Gmail/Sheets) aren’t in the project management tool business.
But that customer’s Job to be Done is still to organize the information and communication on their project.
So, you’ve effectively expanded the pool by switching someone outside the traditional industry confines.
tldr; just comes down to how you define “zero-sum”.