What were the alternative you were considering?
Slack is a healthy marketplace and I think buying plugins is something most users know they can do and take pleasure in. Shopify also boasts how mosts users adopt apps and it's core to their user experience. CloudFlare, I have no experience with their apps, but as a CloudFlare user myself, I don't think I've ever felt compelled to browse their app store. Perhaps some good 2 cents to keep in mind? On the surface though it looks like they basically can do plugin/widget style stuff, where they can inject your script into the markup of a page before they serve it to the user. Zero-integration steps for the user. Similar to how Shopify's ScriptTag API allows zero-integration steps for widgets, etc.
For me personally, I really weighed between Shopify and just rolling my own authentication and doing normal advertising/customer acquisition. As I state in the article, I just really didn't want to do any of that, even on a small scale. That means you're constrained to the problem domain of ecommerce, but that there is a deep well to pull app ideas from. I'll probably end up doing some Shopify App Store ad campaigns consistently and Google Ads if customers find it solves their problem and has traction though!
I also see a ton of YouTube ads for Shopify Apps, so I imagine that is a working channel for app developers. Only one way I'll find out for sure -- testing! :)