There is likely an IT platform angle to it. Simply put, the old MF platform may have been implemented on older technology that is becoming more expensive to maintain and is less flexible from the standpoint of integrating with new technologies. A simple example, without getting into the weeds too much, is stuff written in COBOL vs. Java or python or other modern languages. It is much harder to find COBOL programmers, and to upgrade code to support new requirements (most of the time a more modern front-end is applied, but that can only go so far). You cannot just "migrate" COBOL to modern languages, you need to do a rewrite. And this may be code that over decades has become "spaghetti".
The newer platform might be more flexible and easier to maintain and add features to, using modern programming and virtualization/application subsystem technologies. You also do not have to maintain staff for both the old and the newer platforms, as few people understand both (I did, which helped keep me gainfully employed at Megacorp
).
Those things are likely reasons why for them it is more cost effective to get rid of the old platform, rather that just charge folks still on it more.