Agreed, there are different solutions that make sense for different businesses. The smaller your margins, the more important it can be to run your own stuff. Or maybe you just want to cut out a layer of someone else's profit from your own revenue to give you more flexibility in your business. If you can get costs to $5/unit at the same margin when everyone else in your industry is $10/unit, that can be a big motivator.

It also depends on how your service is positioned to grow. If you have any sort of reliable, foreseeable growth then you can save money by building infrastructure yourself. You are paying for Amazon's overhead to allow you to spin up any instance anywhere at any time (and there's no guarantee it will really be there) -- even if you're buying hardware 6 months ahead of when you actually need it, as long as you're growing and not missing the mark too badly you can do a better job on cost.

Most importantly: are you planning for success? If you sign a giant customer, will it help you grow your margins by getting you to better economies of scale with your vendors and your expenses? Or will it crush you because now your burn rate just doubled? There are economies of scale available with AWS, but they aren't great and you have to be pretty huge to get anything meaningful.

Two of your examples stuck out to me:

Dropbox was never entirely run in AWS. That complicated things operationally to a degree, but ultimately made it a lot easier to move pieces of the service in or out of AWS as it made financial and technical sense.

Pinterest was built in AWS. Since they waited so long to start seriously considering alternatives, they're in a tough spot. Migrating out will require a substantial capital commitment (not to mention engineering and product commitments), and they'll have to pay for both environments during what is liable to be a lengthy cutover period. You have more resources to leverage, but the scale of the problem just gets harder over time.