I see this common pattern where a previously private infrastructure is opened up (usually from low abstraction), and the ecosystem is split into an open base and a private thin layer, and that private layer might just reimplement the same tradeoffs that the incumbent private monoliths made.

Examples being Git/Github, Crypto/Centralized Exchanges, and as per the topic, email.

But I think that it's an important distinction that the base infrastructure is open, and that technically an incumbent could join the fray, albeit with a lot of catching up to do, and mix it up.