You can give feedback and use a prioritization mechanism together.
"Hey, that's really difficult right now, because ____, so we'll put it in the queue for some future time."
"I'm not sure how exactly we can do that and still <do other important thing>, because ______. It's in the queue so we can think about it later."
"Doing _______ seems to create a significant problem in ____________. Maybe there's a better way. In the queue."
This way the person doesn't need to be told an outright "no" or back down; they can save face and have time to change their mind. Similarly, the idea is preserved so that if these reasons weaken or we come up with a better idea about how to do it, we can move forward.
Finally, you preserve the concerns about the idea. Then, if you're not there when the idea is brought up again, then they can encounter it already-in-the-backlog with the concern/problem.
"Hey, that's really difficult right now, because ____, so we'll put it in the queue for some future time."
"I'm not sure how exactly we can do that and still <do other important thing>, because ______. It's in the queue so we can think about it later."
"Doing _______ seems to create a significant problem in ____________. Maybe there's a better way. In the queue."
This way the person doesn't need to be told an outright "no" or back down; they can save face and have time to change their mind. Similarly, the idea is preserved so that if these reasons weaken or we come up with a better idea about how to do it, we can move forward.
Finally, you preserve the concerns about the idea. Then, if you're not there when the idea is brought up again, then they can encounter it already-in-the-backlog with the concern/problem.