I think railway or construction companies do just create some corporation for each project, so it can't be illegal per se. What's going to stop you from just creating a company for each project (splitting when projects become to large). You can also keep a reasonable number of companies around and just allocate tasks and resources as necessary.
I actually believed that having a ton of subcompanies, IS already how any larger company operates.
You have tax and management overhead for all of these companies, it's not free, and certainly costs more than just using the enterprise plan. Plus Anthropic can sue you arguing the corporate structure is just a workaround to avoid paying the enterprise plan, and you'd probably just settle out of court anyway and have to switch to the enterprise plan.
The reason _some_ (but certainly not all, or even the majority) large companies break up into subsidiaries is usually for liability purposes (so one mistake by one part of the conglomerate can't expose other parts to the full risk).