Software is hardly ever written from scratch. That's not cost effective. You will throw away a lot of what you've learned over the years. You will force everybody that will work with your engine to be trained anew from scratch. You will have to get all the little bugs out again.
Just realize this. Windows 8 will still have code from MS-DOS in it. Most of it might be touched a zillion times since the eighties. But I can guarantee you that there will still be algorithms and literally lines of code that have survived that long. The majority of Windows 8 will come from Windows-NT (which is over a decade old), but there will be even older code in it. Who cares ? As long as it works.