[GianFabien]

The legacy systems you refer to are used because they satisfy the business needs.

But many have awkward, confusing UI are brittle in face of required changes and the documentation out of sync with the code which often has confusing and contradictory comments.

The management is always screaming for the latest fix, change to be made ASAP. So the programmers do their best in adverse conditions. Any attempt to do a proper job is a career limiting move.