Conversation for Possibility Killers

Here's the scene:
Someone is talking about a new idea, a possibility that they are excited about... in mid stream someone steps in...
Yea but...
What about...?
That'll never fly because...
Have you thought about...?
Pretty soon others join in with their questions, concerns and considerations .
And, little by little, what started out as an exciting idea, a new possibility, at least for the one who thought it up, becomes fraught with obstacles and problems - if not flat out unrealistic, infeasible and unworkable.
If asked, most people would say their questions and concerns are designed to help, to expose weaknesses in the idea, or to contribute things that the idea generator had not taken into account. What actually happens though, more often than not, is that little by little excitement and enthusiasm is tempered and possibility is killed off . Especially if the new idea is a significant change from the way things are usually thought about, or the way things are usually done.
Pretty soon the conversation is about: getting back to reality, to what is known, familiar, tried and tested.
Yet paradoxically we are most alive, most turned on, when we are generating new ideas, when we are engaged in conversations for possibility.
So my next question is how do we keep possibility alive? And how do we raise questions and concerns about a new idea - without killing it?