While it's frustrating to be "locked out" during such server downtime, I hardly think it amounts to much time at all measured over the course of a year. It does always seem---to me at least---that server downtime always takes place just during a day/time that I "intended" to play. Of course, "intention" means I wouldn't be distracted by something else IRL and probably the majority of the time my "intention" would not have turned into gameplay.

Forget pursuing legal action. While I haven't bothered to look at the EULA I'm sure it included ironclad provisions protecting Sony from liability in the event of downtime for maintenance, emergencies or whatever other reasons come along to interfere with gameplay. The biggest potential for liability was when Sony got hacked and customer personal info presumably got into the hands of nefarious unknown parties and as far as I have heard no legal action, "class" or individual, was ever undertaken against Sony. Anyone can sue anyone at anytime for anything but that doesn't mean it happens a lot...