We're on a core-based license (love it) so I'm not very familiar with the other licensing forms, however, that doesn't sound right to me.
Is this "Customer Success Manager" a Tableau employee or someone employed by your company?
If a non-Tableau employee then contact Tableau directly. You should have a Tableau sales consultant and/or account manager.
If it is a Tableau employee then get details about it. If you're getting push-back then go above them to the next person in the chain of command. Be polite but firm and provide details.
Thanks for the comment, Toby. Believe it or not the "Customer Success Manager" is my Tableau representative. I was a little surprised when I spoke on the phone to my rep and he essentially gave me the company line with no acknowledgment that this seems pretty unfriendly towards customers. It has been escalated, but I haven't heard any response yet.
It might sound like a hokey excuse, but it is quite true that if you had more Users defined in the Server than the Max Users enabled by the license, the Server could go into a major and unrecoverable failure.
Downgrading a Named User license is not something the licensing scheme was designed for. To go smaller, you'd be better off to look at buying a new smaller license (as opposed to trying to downgrade in place), which would probably overwhelm any possible cost savings.