The limits on third-party apps are legal, not technical. It is not technically challenging to build an application that collects Facebook credentials and then presents alternative views and features. It could, for example, finally be possible to see a time-ordered view of your friends' posts (Facebook doesn't allow this since it reduces engagement).
The development of such applications would serve as a threat and check on the market dominance of Facebook. A popular third-party application could consider adding its own features that Facebook does not have. It would also reduce Facebook's revenue.
What stops this? In the US, it is primarily the CFAA (https://en.wikipedia.org/wiki/Computer_Fraud_and_Abuse_Act). Once Facebook formally tells a company to stop accessing their servers, they are in violation of federal law if they continue to do so.
It seems premature to pursue trust busting action while we still have federal laws that encourage and cement the dominance of a single provider.