I worked on a project that had to recompile some 200+ programs (written more than 20 years ago, in the 1980's to 1990's) because the compiler they were compiled on, became unsupported. We really did not have to do this but we did it just to be sure that all programs will work with the new compiler. The result: all but less than 10 programs required any changes. These changes were mostly adding initialization statements to variables and probably a couple required some small changes to the code logic.
What They Do Not Tell You About 'Modernized' SystemsContrast this to so called modernized systems that were written to replace these mainframe applications. Some had been re-written already only after 15 years of use. (Did I mention that the systems I support had been running since the 1980's?) There are some DBase programs, running in Virtual Machines under MS/DOS and some Visual Basic 6.0 programs had to run under Windows XP virtual machines. One question to ask is how easy is it to find DBase programmers now?
This is one thing those who sell services to convert mainframe applications should answer: Will their system work the next time the desktop or server operating system or database or other system software is upgraded? Will their compiler be supported after 10 years or when a new operating system is introduced? Will their programming language even exist after 10 years? How much effort is needed to change the program to make it work in a new environment?
What About the Servers?With a lot of servers supporting various applications, it is a nightmare to maintain and support. The mainframe is basically just one box and all processes run in it. This was made apparent a few years ago when our mainframe was moved to another physical location. The physical move took two weekends. By Monday of the second weekend, all the applications were running and everything was back to normal.
Think of moving thousands of servers. It will take years to move them from one physical location to another. How much would that cost? How much effort and coordination is needed?
Many people tend to be blinded with new and shiny things, thinking newer technology is better. They think these new technologies will give them better functionality with bells and whistles. While it may be true that the cost to initially develop these new systems may be cheaper, the technologies they use may prove to be unpredictable which in the long run may cost more to maintain if they could even be maintained.