> This may be true, but because there's basically no on ramp to running on a mainframe
You can get a partition of IBM Z to run in the cloud. The cost is about $5/hr for the smallest configuration.
> Splurging on network infrastructure with multi-chasis redundancy is an easier step to take to get to a more reliable system than building against a totally different system architecture.
Yes and no. If you truly need that redundancy than the mainframe is going to provide a much better version of it. SYSPLEXs and LPARs are some insanely powerful technologies.