He could reply to them, or be silent. I don't know why it's so deranged to "take your toys and go home" aka stop using a service (even loudly), if you find that a change has been made that is a dealbreaker for you.

The defense that you can only half-participate in g+ is not much of a defense because 1) people may not want to support changes of that sort with their patronage lest it continue as a trend, and/or 2) the thing being objected to was a forced escalation of participation - why would a user assume this would be the last and that half-participation in g+ will remain an option?