I'd have a long, serious talk with them. It sounds like they are pretty comfortable at the level you have achieved. Maybe they don't want to ever go to the next level. But you have to give them a chance to tell you what's going on. There are lots of reasons why someone may not want to marry. They are the only ones who can explain themselves.
The thing is, if you suspect your partner will say things that sound good but they are shining you on, you have a deeper problem than lack of commitment: lack of trust. If your partner doesn't trust you, or gives you reason not to trust them, the relationship has a core flaw. Once trust is lost, a relationship is irrevocably ruined for me.
It's hard to give up five years. That's a lot of time and emotion invested. But what can happen is a person will hold on because of that investment, and then more years are invested, and it becomes that much harder to move on.
Definitely don't sit and wait. Take action. Talk. Evaluate the trust you have in your partner. Be brutally honest with yourself. You know what you need to do.