> There's a reason Netflix is still on AWS.

I would counter with stack overflow which has scaled great over all this time on only a few self-hosted servers.

The trouble with replacing the buy with a diy later on is that it will now cost at least twice as much to build, because 1) you're maintaining the existing system which takes some of the people most familiar with the problems of the existing system, and 2) the existing system will still evolve a bit because that's business, so you're also trying to hit a moving target.

I think other posters have it correct, you have to think hard about how central a particular feature or service is to your business model. Chances are, differentiating yourself means off the shelf solutions won't be a perfect fit for your core revenue, but all other supporting services should probably be off the shelf.