> We have an email server monoculture.

The issue goes a lot further: We have a technology monoculture.

Technology has this annoying property to trend towards a monoculture. You can also see this when you count the tech giants who hold 80% of whatever product/service market. Every tech company attempts to lock you in, sometimes using the network effect (i.e. social networks), sometimes using tech integration (i.e. Apple).