First off, a friendship is a relationship.
Now, I assume you mean monogamous relationship?
You seem to assume there has to be conflict, that they will cease talking..
I know some older adults who have been friends for decades, and have been romantic with each other at one stage or another.
This whole "ex's can't be friends" is false.
It depends on the level of attachment the 2 parties hold to the monogamy of the relationship, and their desires.
For example, the difference between 2 (old) friends becoming lovers because the level of love/attention/whatever it is from the platonic friendship is not meeting their requirements would be different to old friends being romantic because of good synergy and a mutual desire for romance.
Yada yada yada blah blah blah
Yes, ex's can remain/be friends.
Yes, romance can end friend/relationships.
No, relationships aren't permanent.
No, it doesn't matter what a stranger on the internet says.
Yes, go kiss your friend.