Why would it need to be per user? An environment of that size could easily support many users. Developers spend most of their time just staring and only intermittently need to build. There is no particular reason why everyone would be building at once. Most of the time you'll get all those cores to yourself even though there could be dozens of devs sharing the box.

We’re coming around full circle to the old days of thin client terminals to a beefy mainframe under the corporation’s control. Only it’s hosted in the cloud and all your data is at risk.

You can run it not in cloud. Uber uses its own DCs (although not sure they do that for dev). It’ll probably be more expensive because you can’t scale everything down for the night (unless you have dev teams spread globally).

Also, smaller company can probably go a long way just on codespaces. If GitHub can, 99% of projects out there likely can too.

Does smaller companies have mono repos that size?

Correct. My last job built a low rent version of this and it was very painful. 15-45 minutes to launch a pod that would have taken 3 minutes under Compose. Pushed a new base image? Whole thing would roll over and need another 15-45 minutes.

To do this well it takes manpower. There are some off the shelf options out there for remote/prod like Kubernetes based development that are smarter choices than building your own. The table stakes for doing this is being a big corp that can afford to spend millions on one custom tool with a hope for positive ROI. For the rest of us, find something that works that is open source or easily licensed.