I would have thought it's to-do/kanban or note-taking apps (but with some One Weird Reason they're different and about to take the world by storm). Which interestingly enough is not at all like the reasoning here - they're pretty sticky, or at least until you move on to try the next one - there's just so many of them you're going to need much bigger scope to even start to think about making any money at it. (e.g. Notion has legs, but it's both to-do/kanban and note-taking, and also database/CRM sort of thing. All the others I can think of are minor pieces of a much bigger pie - Atlassian, Microsoft Office or whatever Wunderlist fell into, Github, Gitlab, ...)