The Economist (Jan 20, 2018, Silicon Valley, we have a problem) has the beginning of an answer to my question.
It suggests that tech firms “keep alive businesses that they might otherwise scale back” – as Google has done with its social offering Google+ – as a “way to look less monopolistic“.
It’s a way for a tech giant to show that others are under attack, and that no rival is too well resourced and entrenched to take on.
For now, I’ll call that the “mirror effect“.
The argument does not go as far as to explain why firms introduce me2 services though.