While I agree with your arguments, our digital footprint is the closest thing we have to mind reading and governments will simply not back down on warrantless surveillance, therefore I believe a line is being crossed that shouldn't be.

On the other hand, if you're not doing client side encryption, preferably offline, then you're not protected against a global threat, period. This is because, even if FastMail refuses to cooperate, well funded security agencies can do MITM attacks in spite of SSL/TLS.