Beware the wolf in sheep's clothing.
APIs are hardly new. Early computer systems had "layered products" - teleprocessing monitors and database systems for example - that allowed "companies to access functionality supplied by others". Everyone who has used an e-mail client has used an API. HTTP is an API itself. Nothing revolutionary in principle to see here.
Arguably, the least important thing about the Google Maps API was the API. Mapping applications existed before Google Maps: what stopped them being more widespread was the cost of licensing the maps. What stopped the applications being interchangeable between different geographical regions was the subtly different projections used by different mapping organisations. Google Maps only enabled "a whole host of innovations" because Google sucked up the cost of mapping the globe and produced a global mapping resource using the same co-ordinate system which it allowed people to use at no cost. The API is simply noise compared with the economic transformation - except that it's the mechanism by which Google retains control over its investment.
And that's the key thing about the new API economy - you hand over your data and possibly also a payment and get some service in return. But you lose control of your data in the process. While many of these service providers may be benign, they're going to find out a lot about you and your competitors and that could put them in a position of significant power. And, of course, the more third-party APIs you're using the more vulnerable your business is to a technical or financial failure of any one of them - and you're on the hook for future price hikes unless you're prepared to go through the pain of regular service migrations.
And as soon as anyone can build a solution by plugging together a few building blocks there's no intrinsic value in it, it's simply a race to the lowest margin.
As for the car analogy, well, all those standard parts have led to just 14 manufacturers dominating the world's automotive industry. So maybe it's all down to money after all.