Actually, supporting legacy systems is a VERY lucrative job.
Businesses most certainly grow to large to even begin consider updating systems. Do you have any idea how many mainframes from, say, the 90's, are still used in some of the largest institutions in this country? Why? Because so much of their business flows through the systems it is disastrous, if not fatal, if there were even a single day of disruption.
What is the one constant with new systems? New bugs to be worked out. A good friend of mine does software consulting for one institution for which about every single day $1+billion dollars flow through the software.
It's all fine and dandy to scream upgrade at every opportunity, but that just means you're not involved in anything mission-critical.