I agree that there is no one right answer for all cases. Companies have definitely implemented their own datacenters poorly and been trapped as a result. You can avoid that fate, but you have to hire right and plan right.

In my experience, the difference in personnel between a service built in AWS versus one built in datacenters isn't wildly different. If you have a five person team for the former, you might need a six or seven person team for the latter. You aren't dealing with hardware and vendor problems in AWS necessarily, but it's also not as simple as 'just run your app and forget the rest' (at least once you start to scale up).

Again though, this is if you're smart about how you build the datacenter. There are a lot of tricks of the trade and a bunch of gotchas that aren't necessarily apparent until you've experienced them for yourself. Hiring experience here is the key.

While you don't necessarily want to limit yourself to the lowest common denominator of cloud providers or your own hardware, any time you make use of something unique you need to make a decision. Is it worth getting locked in? What will you do if that solution goes poof in the night?

To use a real recent example: what do you do if AWS runs out of X1 instances and you really really need some more right now to do your work? It sucks to get into a situation where you have no control over the outcome and even money can't fix it, either in your own datacenter or in AWS.