The set of shareholders who would prefer a company return profits / invest in existing products vs risky investments in non-core-competency areas is certainly not empty. If I wanted to invest in a dogfood company I can- I invest in Atlassian because they make developer tools.
I disagree with this completely. Atlassian is in the business of collaboration tools. They've done that for developers, but it doesn't mean they're only capable of making developer tools.
Also, where's the risk here? Other than the inflated price I don't see any risk. Trello isn't just an idea but rather a real product with a massive team and a ton of users who love it. They've iterated on their platform enough so I imagine many of both the technical and experience aspects are polished enough. What am I missing?