This isn't something you can get a blanket answer to. Every situation is different. I've had many times when someone moved over for me, and, many times when I had to (gently) shove someone out of my way.
It depends on a lot of factors. How much faster than you they are, what's at stake, and how long you've been holding them up. If they aren't faster than you by a large enough amount to get a clean pass, you're in the right to defend your position. But if you're having to block their run coming out of every single turn, you're just being a sore loser and absolutely should move over.
Plus, you never know who you're messing with. Not everyone is like me and will put up with being blocked for 5 minutes straight. Some will put you off the track and not think twice about it. Ask yourself; is it better to let him pass and take 3rd place, or, risk him taking me out and being in last?