No, it isn't. Do an internet search on "Service Level Agreement" (SLA).
Availability, generally referred to as uptime and usually expressed as a percentage, is part of the SLA a service provider and a customer have. High availability (like "five 9's, or 99.999%) commands a premium price.
You're not paying for high availability here. Instead, this is what you pay for, per the FINAL FANTASY® XIV User Agreement:
emphasis add.Access may also be temporarily suspended in whole or in part, without notice, due to emergency repairs, fire, flood, explosion, war, strike, embargo, governmental action or failure to act, the act of any civil or military authority, act of God, or by any other causes beyond Square Enix's control, or any other reasons for which Square Enix, in its sole discretion, deems that temporary suspension is necessary. You acknowledge that disruptions of service may occur and waive any causes of action against Square Enix in any way arising from or related to any such disruptions of service.
SE doesn't offer a high-availability version of FFXIV, and IMO given the cost of providing it (duplicate hardware, extra staffing, etc.) few people would want to pay for it.