I really dislike the fallacy that just because you're buying something it means that you're not building anything. In practice this is never true: there's always some people-in-your-org time cost of buying something just as much as there's some giving-money-to-other-orgs cost to building something. So often organisations wind up buying something and spending way more time in the process than it would cost for them to build it themselves.

With AWS I think this tradeoff is very weak in most cases: the tasks that you are paying AWS for are relatively cheap in time-of-people-in-your-org, and AWS also takes up a significant amount of that time with new tasks as well. Of the organisations I'm personally aware of, the ones who hosted on-prem spent less money on their compute and had smaller teams managing it, with more effective results than those who were cloud-based (to various degrees of egregousness from 'well, I can kinda see how it's worth it because they're growing quickly' to 'holy shit they're setting money on fire and compromising their product because they can't just buy some used tower PCs and plug them in in a closet in the office')