How to convince system guys that you dont have to span vlan everywhere?

Hi everyone, I'm a long time reader here but I'm never posting anything because I'm not very good with English. Sorry about that first.

I just change my job(about a month) and now responsible for designing, implementing, and operating network part of new data centers and campus network project for government sector.

The problems are I cant convince server guys here not to span vlan everywhere from data center to campus part. I tried to explain why L3 is better and easier to manage but they told me they already have too many vlan(thay have less than 50) and managing hybrid mode next-gen firewall is easy enough because of GUI.

And last time we spoke, they want to span vlan from switches to NSX domain. They told me I dont have anything to concern because NSX is their job. How do you guys approach this situations or am I wrong that I want to segment the network at layer 3?

Thank you.