Define successful: I 100% live off it ($50k/yr before taxes). Biggest new add-ons are often agencies who can afford to sponsor their development because it's a customer funnel for them; I'm one of the rare new who built a business from scratch and lives off it.
The Atlassian APIs could be difficult[1], but the reward is great: Being a vendor introduces you to the biggest corporates without having to be referenced or pass the Purchase Order process, so you can very easily sell to companies similar to Samsung, HSBC, Defense actors or Ubisoft.
My advice: Build a real product with your add-on, not just a tweak to Atlassian's products. Tweaks = SQL reader, theme, formatting of mathematical expressions, ... Products = Balsamiq, Gliffy, time management solutions, architecture/CAD solutions, requirement management, accounting, aeronautical check-lists, etc. Be a bit ambitious and you'll be the reason why people switch to Atlassian and money will pour onto you.
[1] Difficult = They're scattered between Server and Cloud apis, and architecture is widely different bw JIRA and BitBucket, but it's still possible to start quite fast with https://connect.atlassian.com .