Heroku, being on top of AWS, doesn't really have the greatest uptime, do they? There are plenty of dedicated hosting and VPS companies that have better uptimes than AWS at this point.
Naturally, if your startup is a backup service or something else where having as close to 100% uptime as possible is a truly key feature, then it's probably not in your best interests to rely on Heroku/AWS.
If you're running a heart-transplant registry, you probably need 10 nines, if you're running a cat dating site, you probably need less. Sure everyone wants massive uptime, but few are willing (or even need) to take the steps needed.
Even Gmail goes down sometimes.