Comment by skissane
> The technology itself makes it impossible to give read only access to such systems.
This isn't true. Mainframe COBOL systems commonly store data in VSAM files, or DB2, or IMS, or sometimes some more obscure non-IBM database (e.g. CA/Broadcom's Datacom/DB or IDMS, or Software AG's ADABAS). But whichever one they use, there are multiple ways of granting read-only access.
For example, if it is VSAM, you can configure RACF (or TopSecret or ACF2) to allow an account read (but not write) permission to those VSAM datasets. Or, you can stick DB2 in front of VSAM (on DB2 for z/OS, CREATE TABLE can refer to a pre-existing VSAM file, and make it look like a database table), and then you can have a readonly account in DB2 to give you access to that database schema. Or, there's a lot of other ways to "skin this cat", depending on exactly how the legacy app is designed, and exactly how it stores data. But, probably this is already implemented – most of these apps have read-only access for export into BI systems or whatever – and if it happens for whatever reason not to be, setting it up should only be a modest amount of work, not some multiyear megaproject.
>Or, there's a lot of other ways to "skin this cat", depending on exactly how the legacy app is designed, and exactly how it stores data. But, probably this is already implemented
Given that neither of us knows the actual systems in question, what is more likely, that it's a well-designed system or one that has organically accreted over time? It seems like you tend to believe the former, and I the latter. I suppose my view is based on the fact that, like in statmech, you enumerate all possible systems that can do a particular job, the vast majority of those solutions will not have any organizing principle and will not be amenable to surgical analysis or change.