> The change is necessary if the system unmaintainable, buggy, or can't be easily interfaced with other modern systems.

Unfortunately, all of these are frequently true of legacy systems. Only a small number are relatively bug free. Most are simply unmaintained. Few are easy to interface with.

I have no particular problem with old software and hardware. But the massive expenditure leading up to Y2K is demonstrative of the legacy problem. Let it rot until an impending crisis forces your hand, costing far more than if you had simply maintained it.