What you're basically saying is that Google used open source to create a protective ecosystem that prevents other vendors from taking over with their own moats.

What you're basically saying is that Google used open source to create a protective ecosystem that prevents other vendors from taking over with their own moats. Chrome serves the same function. It ensures Google is not blocked by browser default search engine changing, it prevents other people's walls. But would you claim Chrome is "vendor lock in"? Android is primarily a defense mechanism. If Apple and Microsoft together had nearly 100% of the market for the mobile web, they could change the default search engine to Bing and sink Google overnight. Android if anything, preserves choice and prevents lock-in, because unlike iOS, it is an open OS, and vendors can always change the defaults.