It's a multi-layered problem, but the common sense approach of going where the audience is now has some merit, but there is also the strength in becoming the bigger fish in a smaller pond. Release a client for Instagram on iOS and you'll be up against the official client and a number of third party options. Windows Phone? Well, hopefully Instagram hurries up over here, in the meantime it's virgin territory for a developer to stake a claim - hey they might just get bought by Instagram as a shortcut for the network, and a nice return for them.
I also think that Baxter-Reynolds puts too much emphasis on developers in his argument. Part of his closing argument is this; "Clearly then, it doesn't matter what we as developers do. The market's going to do what it fancies with or without us."
An interesting viewpoint, but the decision on which platform to develop for isn't one that can be made on a wholly empirical analysis - there needs to be hunches, gut feels, previous experience, and if the developer wants to be involved with a system. The picture is far more complicated than shown here, but if you want to boil it down, try this old war horse of a phrase.
"Follow the money."