protect carpeted stairs?
Scotch guard will greatly increase the stiffness and how it holds up to dirt and abuse.  I suggest a heavy heavy layer of scotch guard once a year right after it has been shampooed (let it dry of course).

Long-term carpet might not be ideal.  You need to try to figure out what is under the carpet.  Almost all of the time you can take out the carpet tacking, sand, and the stain/paint the stairs.  This is not hard to do, doesn't cost much, and it is a true DIY project.