> if you assume the same level of organizational competence at running your own infrastructure
I'm not sure that the competence required is organizational (e.g. people managing) so much as operational (e.g. best practices) and even technical, at least at sub-FAANMG scale.
> it isn't easy and it isn't cheap. It certainly can't be taken for granted.
I agree that it can't be taken for granted, but I disagree with it not being easy and cheap. Rather, combining the two, I don't believe it's necessarily hard nor necessarily expensive.
It just requires finding someone who both still has the competence, is willing to use it, and is willing to train others. Running your own infrastructure isn't actually difficult or complicated, but it's certainly not "sexy" and can be a bit tedious at times. That means it's possible to hire inexpensive, less (overall) experienced staff and have them handle that portion. Unfortunately, the "unsexy" part means finding someone to do the training, as well as the actual work when necessary, can be challenging, even though we're out there.
Even then, that's only necessary at substantial scale. In What can get astronomically expensive is outsourcing the wrong things, though that ends up being a form of not actually running your own infrastructure (yourself).
Anecdote: I recently had a phone interview with a startup that moved from "hardware" to the cloud and the main reason cited was the inability to ramp capacity up fast enough (nor predictably fast enough), which seemed odd to me. One example of unpredictability of lead times involved a new server underperforming due to mis-applied thermal compound between the CPU and cooler, which I have never experienced [2]. I didn't ask the rhetorical question, "how could you have picked such a horrible VAR?!" Carefully re-reading the blog post about their transition gave me my "aha" moment: even though it's a company in the SFBA, their datacenter was out of state (maybe not even in a tech hub city, but it didn't specify). They were outsourcing the actual installation, running, and maintenance of their hardware to someone else, far away.
[1] for lack of a better term.. i.e. anything that an IaaS cloud provider would eliminate, including purchasing and vendor negotiations, colo space, network hardware and providers, hardware monitoring, and data destruction
[2] well, OK, that's a lie, since I've experienced it when I've personally done CPU moves/swaps/upgrades in exceptional circumstances, when I was out of practice, but I knew to test my work and caught the problem immediately. I've never had it happen with professionally-assembled systems, presumably because CPU coolers tend to arrive with the thermal compound pre-applied.