Or when having old legacy systems start to impact the company’s performance.

These old systems take more and more resources to maintain, also finding outdated hardware to replace them, not to mention the time and effort of your tech support team.

Any system that requires pre ntsp3 is a poorly designed system. There is no reason that you should not be able to take your old NT based system to the latest available NT SP just for compatibility issues alone!

And yes go ahead and migrate to a linux based solution, i am curious to see how a linux migration would be cheaper than your current windows legacy system.

You have linux software packages that will replace those windows software packages? No, got a programmer on staff to write them? You prepared for a complete in house programming solution to support a 100% custom linux environment like you are describing? I don’t think you do, because if you did your old legacy programs would not have compatibility problems with today’s xp based systems. If you in-house programming solution is Bob the programmer, when happens when Bob leaves, and the company finds them self in and even worse position than they are now?

Are your tech's really able to support linux at the level needed for a migration like you are taking about to take place, we are not talking about desktop level computers.

Linux is not "free". You pay for IT one way or another.