I've casually watched F1 on and off over the last 20 years, so I'm not a "hard core" F1 fan. I didn't like Fernando Alonso early on, he came across to me as a prima donna like many F1 drivers (a plague not limited to F1 in big time racing, for sure). But in recent years, maybe due to his struggles, I've come to appreciate his talent and drive to compete. I think the guy really does love racing, and wants to enjoy it. If he goes to Indy Car, I think he'll have a blast and the series and fans will welcome him with open arms. Assuming he goes to a top team (there is no doubt), he will have a shot to win every race. He will show everyone just how good he still is, and perhaps draw some deserved attention to the other drivers in Indy Car who will give him all the challenge he can handle. Indy Car has been gaining momentum the last few years, I only see this as adding to that.
Also, there is a lot of room in the Indy Car schedule for additional events as has been mentioned. Several Indy Car drivers run the Daytona 24 hours, Sebring 12 hours, Le Mans, Petit Le Mans... hell maybe while he's over here he might even be offered to try Nascar. Before you scoff at that, Nascar road races can be wildly entertaining and he's shown he's a quick study on an oval. Has ANYONE ever won Monaco, Le Mans, Indianapolis 500, AND Daytona 500? A.J. Foyt has 3 out of 4!