It would not be hard at all I imagine. I built this as a solo founder so the team at Trello could no doubt replicate this if they desired. This is the risk with building anything on someone else's platform and I don't deny I am taking a risk.

However, I'm gambling that the benefits outweigh the impact of those risks in this situation. Building Corrello on Trello gave me access to a large user base (5M as of last autumn) and one which is growing. Check out google trends for asana, basecamp and Trello https://www.google.co.uk/trends/explore#q=trello%2C%20baseca... Not directly linked to their user numbers but Trello looks to be increasing in terms of awareness. I believe there are a large amount of people out there who were previously managing projects in excel or on post it notes. Trello has pulled those projects into the cloud and put a decent API on them. That's a whole lot of projects which were previously stuck to peoples desktops (literally in some cases) which are now accessible via an API. I think it's worth the risk that Trello decides to replicate this functionality as there is a good opportunity there if they don't (or until they do)