Over time, though, your needs change. That automated test suite is large and getting larger... and now you have a whole new project that needs some time on the infrastructure. Your system configuration is now a two-machine config, and you have to share test systems a bit more. In other words, life happened. Time to catch up.
There's good news and bad news here. The bad news is that you are probably looking at a fairly large project to make major changes to accommodate your needs. To rejigger your test estimates to account for having fewer net test systems will have an effect measured in weeks. To change your automated tests and the infrastructure in which they run to be more efficient will take weeks or even months.
The good news is that you can have about 80% of what you want with a lot less effort. Sure, it'll be a bit cruddy, but it's probably a viable internal solution. But you can find a way. You can reorder your test plan to do non-conflicting things in parallel. You can run just a small piece of the new stuff alongside the existing automated tests.
Some jobs are big jobs. And eventually you're going to have to do the big job. But in the meantime, look to see if you can get part of it with a lot less effort. It'll save you a lot of interim pain.