That was insightful to some point. But, of course, the relevant metric is "expected availability" -- before I decide to go on the service; therefore, not the same as "customers served". If I have to think about downtime then my experience is degraded; moreso if I have to delay and batch planned interactions (all of which will later count as successful!)
[Edit: To the point: a high rate of randomly-timed failures is a kind of degraded experience, but not as critical as blocky patches of downtime. A 1% rate of randomly-timed failures is much much much preferred than having the service go out three straight days every February.]
Also: uptime is not the same as "customer delight". It's all about time.