Emulating even a mid-sized network in GNS3 requires massive resources, and my cisco account manager doesn't seem to even get why I'd want to deploy a test system of 50 different multi-vendor switches (and key supporting services like syslog and tacacs) with terraform, run some tests, apply a configuration change, and run more tests.

And virtual switches aren't the same as physical switches in any case, they have different bugs, different features, different responsiveness.