>Small businesses are forced to do business on their platforms according to their rules, or else they go bust. Programmers are forced to learn their APIs, so that their "app" can live in their walled gardens.

???

What are you talking about? For a typical fullstack app the proprietary bits probably account for less than 5% of the codebase.

>They soaked a huge amount of talent to optimise their ad and recommendation engines.

That's just PR/advertising/sales. If the companies didn't exist it's not like those job or efforts will disappear, they'll be allocated elsewhere, classified ads in newspapers for instance.