If Robinhood opened up an API, wouldn't it be possible to invest in a way that mirrors existing index funds? I'm not super familiar with the details but it feels like it would be.
It'd be possible, but you can buy index funds on Robinhood already.
Other services that are built around long term investing provide additional features like automatic rebalancing and dividend reinvesting. An API could allow you to emulate those as well, but at that point, it seems like you might as well just use a robo-advisor instead of building your own on top of an API.
