Why not use aged/ anonymized data? This way you can use Prod data in Dev with custom security rules anonymizing your data and following DSS.
Dev: "Could we pull an export of relevant historical data and get some time to write code to safely anonymize that, and stand up a parallel production system using just the anonymized data and replicate our deploy there, so we can safely test on real-ish stuff at scale?"
Lead: "I'll think about it. In the meantime, please just build the features I asked you to. We gotta hustle on this one."
I'm not arguing with this hypothetical exchange that it's infeasible or even a bad idea to do exactly what you suggested, but attempting to justify an upfront engineering cost that isn't directly finishing the job is a difficult thing to win in most contexts.
My point above was that it's not necessarily easy to convince the operators of a business that it's a justifiable engineering expense to set up a new "prodlike but with anonymized data" environment from scratch, because it's not a trivial thing to make and maintain.
I do think it's pretty easy to convince operators of a business to adopt the other strategy suggested in a sibling thread: run a dry mode parallel code path, verify its results, and cut over when you have confidence. This shouldn't really be an alternative to a test environment, but they can both achieve similar stuff.
I agree - it's a nice low-risk way of doing things.
It is as low risk as trying to use Windows and Microsoft Word with a keyboard and mouse mirrored to a Linux machine running Open Office and expecting the same results.
You can't run the two systems side by side - different screens, different keyboard entry... and some of the keyboard entry can't touch the other system.
And this is assuming you can put a dry path into the production system. If the answer is "no", then you're putting a dev environment into a production environment... and that's certainly a "no".
We had test environments and we had a lab were we had two rows of systems where the two systems sat back to back and each row was hooked up to a different test store (not feasible in a production store environment).