Equity aside, there's also a lot of logic in his decision to accept an offer from United. Cleverly, the offer was accompanied by one for his former creative business partner Robert Campbell. Both parties know they work well together.
In addition, there's still a lot of entrepreneurial spirit at United.
Kelly and Campbell will relish the challenge of harnessing this energy and returning the agency to its former HHCL glory.
With Andy Berlin as the global chief executive, they know they're not working for the conventional multinationals that they both ran away from.
HHCL has had a roller-coaster ride since its heyday in the 90s. Under the former chief executive Nick Howarth, it effected something of a turnaround.
However, Kelly and Campbell take the helm of a weakened shop. Without Howarth, the prospect of a Sky review is a real one and Pot Noodle will move to either Fallon or Mother.
WPP's decision to buy the remaining stake in United shows, however, that Kelly and Campbell will enjoy the full backing of Sir Martin Sorrell.
They built Rainey Kelly from scratch so it follows that they have the ability to rebuild United, as long as they can rekindle the fires in their respective bellies.