I honestly think the best solution for the Heat as a team is Riley.
He's a strict, no-nonsense, disciplinarian kind of coach. He'll put the players in their places, and I think that's exactly what they need to thrive.
They don't lack talent. They lack cohesion. I think Riley's approach would enforce that cohesion.
Having said that, I don't think Spoelstra should be fired. He's a young coach who helped get the Heat through a couple of very good teams on their way to the Finals, and ultimately came up short against a heck of a team. If I were Riley, I'd want to see how Spoelstra grows from this, and what he does differently to start the next season. If he shows signs of changing nothing in the way he handles the team, then show him the door. But if he shows growth and evolution in his abilities as a coach - and I think he will - then let him grow.