We were already paying colo providers by the MW and had the people to build out the space. It's just that the upfront capex required for a colo provider to build capacity was getting to a point where that meant a significant bet on their part on our continued growth. Thus we started having to look at one options, one of which is to contract them to build bespoke infra.

Regarding cloud being much higher cost: indeed. Anecdotal evidence; I know of a big reference customer who said they would consider the move to cloud a success if total cost didn't go up more than 2x. A few years in, that hadn't happened yet...

Agreed. It's a hassle to negotiate a few racks and that's barely a few millions dollars. I can't imagine doing that for a hundred rack at a times.

Twice as much would be a huge savings. In the cloud, I can give you 20k cores in one hour and I can give you 100k cores in two hours, after the initial investment to open multiple regions and networks backbones. The price is known in advance and the investment can be cancelled anytime.

Obtaining the same 20k cores from a colo will be a hassle. Months of negotiations with nothing being done. I'd expect any sane provider to bail if trying to discuss 100k cores, it's too much upfront investment. It would take years to get half of it, the project is long abandoned before anything is delivered and everyone who worked on it has left.