From the book: "Migrating Legacy Systems: Gateways, Interfaces & the Incremental Approach" (Morgan Kaufmann Series in Data Management Systems; ASIN: 1558603301 - April 1995) by Michael L. Brodie, Michael Stonebraker (Contributor), Michael Stonebraker

  Costs due to problems of single legacy IS often exceed hundreds of millions of dollars per year!
  Failures
  Maintenance
  Inappropriate functionality
  Lack of documentation
  Poor performance
  They are inordinately expensive to maintain
  They are inflexible to adapt (difficult to adapt to changing business needs)
  They are brittle (easily broken when modified for any purpose)
  Perhaps they will one day break beyond repair
  Lack of techniques or technology to fix legacy IS problems

Consequences of such problems

 Apoplexy:

   They consume 90% to 95% of all IS resources
   This prevent organizations from moving to newer software
   Organizations are prevented from rightsizing that involves moving from large mainframe computers to smaller, less expensive that fully meet current IS requirements
   New requirements cannot be developed with the 5% to 10% remaining resources
  Develop new functionalities means extra-resources have to be found, which means calls to extern staffing or outsourcing and so increase costs
   The organization is not fully master of its development strategy since it has to take account the cost of external resources to maintain the legacy application and to develop the new applications

Please read then the practical problems described as a list of claims about such systems, decisions and consequences resulting from the decisions.

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