IMO, based on their current business models, big US tech companies would be better off financially by pulling out of the EU entirely than by allowing interoperability via public APIs.
They will lose most of their users to small platforms if they provide too much interoperability - Not just in the EU but all over the world. Their monopoly over the bulk of the world's user accounts is their only real competitive advantage.
That said, in terms of social good, open APIs would be great.
If they did pull out, that would be probably the largest business opportunity in human history, with a $16 trillion economy suddenly having no large competitors in many sectors.
I'm sure entrepreneurs in the EU are salivating at the thought of that happening.
They can (and likely will) produce a layer of bureaucracy that could keep out any non-EU player if they wish, stuff like having to register with them with EU-based company information before they give you an API key (this is just an example).
They will lose most of their users to small platforms if they provide too much interoperability - Not just in the EU but all over the world. Their monopoly over the bulk of the world's user accounts is their only real competitive advantage.
That said, in terms of social good, open APIs would be great.