Sometimes you don't know if a product is worth being an entire platform until it's been thoroughly field tested. It's too expensive to assume that everything is.
Perhaps in Google's case there could have been more foresight, though.
Yet, every product developed at Google seems to be developed with absolute scalability in mind from the get-go, wasting lots of effort invested into scalability in the case of the product failing to get significant customer adoption.
Since 100% of successful Google products will be large, it makes sense to design for scale from the get-go. But if some of Google's products won't end up being platforms, it might make more sense to bolt that on later.