do you think one's ex and current partner can be friends?

Just like you, I don't think so. This is a situation with multiple negative angles and with trouble written all over the place. The only way I see this happening is if you have any children with your ex, in which case your ex probably has to see you (and them, obviously) with some frequency. But, besides that scenario, I don't think it's possible.

It is depending on the relationship of the both. If they have enough money and much love, they can not be the part they will be friend. If they think about the advantage and dis- vantage they will loose their friendship. You will be no longer happy. It will be like a new cat and a old dog.

sure - it can happen. I don't know that it does very often both people would have to be mature about it