A single computer with 4 sockets of xeons will outperform a mainframe but will have more downtime.
The mainframe has best possible single thread performance and as much cache as possible, redundancy and parts can be replaced while it's online, but not that many cores.
The cost is very high - when I looked, 1 million per year is the baby version with only 1 CPU enabled and no license to run the cryptographic accelerator and limits on software, etc.
Commodity hardware you buy and use for 5 years, so the amount of good hardware you can buy for the price of owning a mainframe for 5 years is a lot.
For the money, you can buy a lot more CPU, ram, network, storage, etc and hire Kyle Kingsbury to audit your distributed database.
At one point, IBM was selling base-level mainframes for $75,000 (see https://arstechnica.com/information-technology/2013/07/ibm-u...)
It is true though that a 'realistic' configuration is likely to cost north of $1 million, and that none of these numbers include the price of the software.
Another is that the software and hardware designed to turn a baby system into anything useful is astonishingly expensive to people not used to dealing with this end of the market. Enjoy finding that your hypervisor is licensed at 5 figures per core, and that your cores are six figures a pop.
Even if the mainframe never goes down, the entire site will go down (eventually the rack power supply, HVAC, fiber, natural disaster, backhoe, etc. will get you even if your CPUs and RAM are redundant and replaced before they fail), and then either your entire business stops or at least processing for that region stops, or your system is resilient to site failure because you built a distributed system anyway.
If you could rewrite your software to be distributed and handle a node/site going down, you could run a single site on 5 servers that together outperform the mainframe (by a lot) and can be serviced on a whole server basis (though of course, expensive x86 servers also have reliability features), or use really cheap hardware without even redundant power supplies, but have enough of them to not care.
The modern solutions are better than the mainframe, and the only reason to use them is management risk aversion and unwillingness to learn new things.
100 miles will add 5ms (round trip) to your disk flush on commit. So a system like this has the sequential and random IO latencies of a RAID of SSDs but the flush (database commit) times of a 15K RPM spinning rust disk. People lived with mechanical disks, it's ok.
Sync disk replication (in one direction) over a fiber line is not an exclusive feature. Having both sides be active, instead of active and hot standby requires some smarts from the software, but modern distributed databases do that, and if you're careful you can get far with batch sync jobs.
For read-only batch computations you can always add some extra redundancy and partition the problem. So, I don't think it is likely that a mainframe would be useful here.