Since I've only been paying for domain registration while using email for free, I feel that I can't complain as much, at least not in good faith.
Still, both the freebies and the paying customers get 6 months to decide which is all I can really ask for. GandiMail has plenty of room for improvement so as long as they can justify the cost increase, I'll consider staying.
I think we as customers have to manage expectations and at least let the vendor process their changes before making our decisions. That's what the 6-month period is for. If thousands of customers take flight immediately after the announcement, the rest of us might end up with a lesser service than we otherwise would've. It has to be subsidized somehow, clearly domain registration fees weren't doing the job.