Re: It's mainframe apps and languages that are aging out...
@Ron, I'm trying to decipher what you mean by "iron clad reliability of the mainframe"? Most of that reliability had to do with the o/s, not the hardware itself.
I think you are talking about the duplicate/fault tolerant hardware that started with mainframes. But most systems now are dual power supply, RAID disk arrays, dual NICs that are teamed, etc. So, in my mind, reliability is much more about the resilience of the o/s itself, which of course follows thru to applications running on it.
Linux is certainly more trustworthy than Windows but still a workstation/single user based system. That was it's purpose, we already had UNIX for servers. But no way it is as robust (and secure) as mainframe o/s or even i5os that runs on IBM midrange POWER systems. For that matter, even the POWER servers running AIX (UNIX) are better, it is a true server o/s.
What happens is people go to reengineer these apps to run on the Linux/Win servers and end up using these cobbled together "frameworks" to get the job done, which normally have more (security) holes than swiss cheese. That stuff brings in the DevOps stuff, where you are updating code with HTTP PUT and DELETE, and then you wonder how hackers are modifying your stuff.
You just don't have those problems with traditional mainframe infrastructures, it takes an insider with a LOT of business know-how to even know what to do. And many of those shops still divide the application guys from actually making changes to production system, usually have very formal change mgmt systems in place.