If you have a romantic relationship with someone and it ends, can you continue on as "just friends"? Is that even possible?
It's pretty rare, I think. It's hard to divide affections. "Romantic relationships" generally have a lot of emotional baggage (for want of a better word) with them. If the romance ends, I think one usually wants to move on or put sufficient emotional distance between the other person so that romance is possible with someone else. (This is one of the reasons why dating co-workers is generally not a good idea.)
Another issue is that often (usually?) in dating relationships that break up, the affection is stronger on one end than the other. That makes it even more difficult to continue the relationship in a platonic way as equals or even "just friends".
If it's a relationship between emotional equals, and both agree on how the relationship should go, then "just friends" can work. But it's pretty rare, I think.
My $0.02.
Cheers,
Scott.