Do you have applications that exhibit one or multiple following traits?
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Poor or no support from Vendor.
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Poor or no support due to obsolete technology and lack of expertise.
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No equivalent package available in the market.
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High maintenance cost due to poor design in the first place.
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Emergence of state of the art technologies may have happened since the application was built and the benefit of such migration outweighs the investment to migrate.
If the answer is yes, the more traits your application have, the more critical is to migrate or re-engine your applications.