As I said before, being needy is unattractive and you want to avoid this at all cost.
In fact, if you do so they will go to the extent of prolonging their rebound relationship just to prove you wrong.
They will soon realize that the new person isn’t right for them and they were just being intimate to quench their thirst for intimacy.
And once they do realize it, they will break up with them.
At this point, even if it was your ex who broke up with you, you don’t want to give them all the power.
You don’t want to be the one who is always available for them. Acting like your life is over without your ex will only lose their respect for you.