Still, things fail. A-tier clouds also fail, and you may still have to design for it. Rule of thumb, if you are capable of rolling out your own version, you'll be far more competent planning for & handling downtime, and will often have full ownership of the solution.
Also, any company with strict uptime requirements will have proper risk analysis in place, outlining the costs of the chosen strategy in case of downtime; these decisions require proper TCO evaluation and risk analysis, they aren't made in a vacuum.
Also, any company with strict uptime requirements will have proper risk analysis in place, outlining the costs of the chosen strategy in case of downtime; these decisions require proper TCO evaluation and risk analysis, they aren't made in a vacuum.