It will probably work for a good number of years, but eventually you'll get to the point where you have to maintain old hardware, running an old, unpatchable OS, in order to run the software. Meanwhile, you're missing out on all of the new framework and language goodies that get developed. Eventually, you'll have a need to fix something or add something that isn't possible in your environment and then you get to pay the entire bill for your accumulated technical debt.
My take: you should already be working on an upgrade to a newer platform or replacement for the application. My preference is almost always to do this before I'm forced to by circumstances.
