$12 million/year for cloud seems like a high enough threshold to go internal but I'm not even sure that's enough money to sway executives away from cloud.
The main issue is that internal IT departments at corporations have slow service from the IT staff compared to AWS/GCP/Azure and its engineers. Corporate IT departments typically don't treat their coworkers in other departments as internal customers. Instead, they treat them as adversaries and a nuisance.
That's why at non-tech corporations, the first group that experimented with the new AWS cloud service were the development teams. They got fed up with IT service backlogs waiting for new dev/sandbox/text servers. They got tired of waiting for IT to requisition a server and then install the os, db, etc. If the programming manager then asked IT to install an Oracle update, the IT department might say "well, Susan our db admin is on vacation till next Monday so we'll get to it then." Those kinds of inter-department interactions were frustrating to the internal customers of the IT department. With AWS, the programmers got their sandbox servers in spun up minutes instead of weeks or months. Once the dev teams were sold, the more risk-averse and mission-critical production workloads eventually moved to AWS too.
The author of this article doesn't seem to understand the dysfunctional relationship between internal IT and their coworkers they serve. It's more about responsiveness and iteration speed of the AWS/GCP/Azure employees' vs the IT employees than expensive AWS EC2 compute power vs cheaper internal racks of cpu.
E.g. The Guardian pays AWS more for cpu+diskspace+network than what they can build on their own. That's the raw hardware costs. But it doesn't matter because Guardian's internal IT staff with an internal cloud stack just can't match AWS: https://www.computerworld.com/article/3427004/the-guardian-g...
To me, it totally makes sense for Dropbox to migrate off of AWS to save money. They're a tech company and have the engineering culture to do it. A lot of non-tech companies (like The Guardian) don't.