Businesses will have backup requirements for many years.  Probably until after both of us are retired...

A quick search on LinkedIn for "netbackup" found 25 jobs.  Expanding that to a web search within LinkedIn found 2,671 jobs.

There's work out there, but some of these jobs are not for people with only NetBackup skills - some/many are combined with system or storage administration work.
 
The average salaries are best found by referring to something like Certification Magazine or one of the many other rags that advertise averages.  You'll be paid at what you can negotiate based on what you're worth.  Why aim for "average" pay?  :-)

There's a career to be had in NetBackup.  If you're good at NetBackup but decide to switch to another enterprise backup product later in your career, that should not be too hard - it's not the syntax that counts but the ability to understand the root cause of an issue and resolve it, and to implement the right stuff to address the business needs.   Always focus on the business requirements and not the technology.