I can imagine we've all thought about it at least once. Many of us have done it. Maybe it worked out & the two of you ended up back together...maybe you just stayed good friends and still are to this day. Maybe it was a nice thought, but ended up a nightmare. My take on it, at least for myself, is don't do it. Here's the why: Feelings do not go away overnight & regardless of what people may say, you can't shut that shit off like a light. Do you really want to hear about your ex's new love in their life? Do you want to be the person listening to them & giving them advice about a new relationship, or listening to the problems they're having with a new relationship? Do you want to be a "safety net" for your ex, sitting on the sidelines waiting for them to pick you up when it's convenient for them? I sure as hell don't. I'm not remotely interested in any of it.
Especially, if your ex was the one who ended your relationship with them! Decisions have consequences. Sometimes good, sometimes bad. Let your ex find out the hard way what life is like without you. Don't allow them to use you as a safety net. If you still have feelings for them & still want a relationship with them, politely let them know when they throw the "let's be friends" bullshit at you that you're not interested in a demotion. Do not give them power over you. Stand up for what you want & don't settle for less than that. It's not as easy as it sounds, but in the long run, you'll be glad you did it!