It's an alternative implementation of many of the same concepts. You don't have to go to IBM and purchase a monolithic box with POWER processors in it.
A mainframe in software pitch sounds a bit like that 90s RAM expansion software. Could it possibly work? To a limited degree, perhaps. Would I want it instead of an extra RAM stick? Hell no!
For example, you can get a pair of enormous 100 Tbps core switch/routers for your data centre from Cisco. Upgrading these is terrifying, they cost an incredible amount of money, and if one fails you lose 50% capacity. Conversely, if you run them active-passive, you waste 50% of your capital investment.
Alternatively, you could use a cluster of 1000 VMs on commodity hardware to run your traffic management, with the rules defined purely in software. This can be an order of magnitude cheaper, scale to infinity, and allow smooth upgrades and change rollouts.
The former is what every large enterprise does, the latter is what the public cloud hosting providers do. You can't argue that it's some sort of "fake substitute" when the clouds operate production networks at a scale that far outstrips anything you'd find in any private data centre!
Similarly, many large enterprises use and prefer monolithic mainframes, but in the cloud the preferred option is something like Kubernetes.
For example, with K8s on a typical cloud, it's possible to use "spot" instances to temporarily scale out workloads at night for batch processing to use tens of thousands of CPU cores. No on-prem mainframe can compete with that on any metric. Not absolute throughput, not price, not price/performance.