[mgkimsal]

Unforunately, it still does creep in.

The 'real business purpose' argument must also take in to account skills of available workers.

If no one is able to be hired to maintain and extend legacy system X, you're left with

* hiring workers with modern skills and backtraining them on older/legacy/custom stuff that won't do them much good in the future market, so they may balk (the smarter ones would demand a hell of a lot more money).

or

* converting to something more modern and perhaps somewhat future proof. The code itself might not be future proof, but the processes the business puts in place during the conversion should take in to account future conversions and upgrades as part of the requirements.