Just because you have an old system and you do things the way they are always done doesn’t meant that there isn’t room for improvement. The fear of “we can’t move we have too much data” prevents you from seeing things in a different light.
You really need to think about: how can we add more value if we don’t change?
Data can be migrated to new systems, by hand or through scripts or programs. The data can always be migrated using scripts or programs that you can test the process and ensure data is migrated correctly. When it’s done by hand it could take weeks and it’s very hard to verify the data once it’s moved otherwise you’re double-handling it and extending the timeline again.
Without moving away from the old platforms and leveraging new tools you will miss out on the business intelligence analytics and reports that can help you make quicker decisions.