I'd dispute that, because of the following:
* Heroku is in no way equivalent to a single full-time sysadmin, not even close
* The choice is not between managing your own hardware and Heroku, it is typically between Heroku, VPS (managed hardware), AWS, and then possibly your own hardware. The same answer does not apply to everyone.
* Many small companies don't have a large wage bill (outside SV), and don't have lots of devs or a sysadmin, instead they tend to have devs who do the sysadmin too.
* Heroku can be significantly more expensive than other options like VPS which also don't require managing hardware (your RAID example) - for some businesses not burning VC money, that makes a big difference. In your example of $1000, you could probably serve the same customers with $100 a month using VPS servers.
* Dynos are not the only cost, you'll be charged for storage too, and $1000/month is not a rounding error for many businesses starting out
* As your business scales Heroku would become more and more costly - there have been reports of clients being charged things like $20000 a month when they start to hit scale.
* They've had more than a few major incidents in this year alone: https://status.heroku.com/uptime - no service is perfect, but that's not once every couple of years
Heroku can look like a bad deal for all sorts of reasons, not least of which cost, and it is not comparable to hiring your own sysadmin, more like having a freelance sysadmin on call who is also responsible for thousands of other sites which are liable to be down at the same time. If you are medium/high traffic website with few other tech costs, heroku can be very expensive compared to the alternatives. Additionally, as documented by some of their customers, they are an extra level of indirection between your app and the user and that make troubleshooting issues or optimising harder -
http://news.genius.com/James-somers-herokus-ugly-secret-anno...
I would disagree anyway though that Docker (in its current form) is going go to kill services like heroku, because it requires significant expertise and setup time to get right, and doesn't fully address the same concerns.