I wonder the same. I guess it's a matter whether both parties want to or not, just like romantic relationships.
For example I used to be close with someone who unintentionally but repeatedly did things to hurt me. How we defined friendship was clearly different. I couldn't expect any different from her. One situation upset me so that I decided to walk away. Its been two years now and I miss her soooooo much.
Since, she's bought a home, went to law school and is flourishing beautifully. While I'm open to rekindling or mending what we had, I think our time has passed quite frankly. It would be romantic to think that somehow it can work but...doubt it. I'm not sure there's a measurable benefit to either one of us.