Insomniac going exclusive with MS was the absolutely right thing for that game.
IMO, the jury is still out on that - and perhaps, we'll never know. Personally, I'm sceptical. Insomniac has always been a well received and wellknown developer on the PlayStation brand. Any new game they bring out would automatically receive some attention and awareness. As an example; I wouldn't be surprised if a large majority of people who owned and liked any of the Ratchet & Clank games also bought a copy of Resistance (quite a niche shooter) for that reason. I'm sure there are many PS users who have known and enjoyed previous Insomniac games that would be quite interested in the Sunset Overdrive game, if it were to come out for their platform.
However, they are bringing it to a different platform, with a different userbase - in that sense, they are starting from scratch. It's only logical that they would need quite a bit of PR for that reason, which is where the exclusive contract with MS will come in handy. But was it the right choice? Will they end up with more sales? I'm not convinced, especially considering that the PS4 offers the larger userbase too (something to be fair, Insomniac couldn't predict when they made their deal).
To the same degree, there are probably quite a bit more Bungie/Halo fans to be found among the Xbox One userbase than PS4 - which is probably why by ratio, there's a higher sale/userbase ratio for Destiny on Xbox than for PS4.