So just for the record, this conversation has mostly been had plenty of times before. Most people agree with OP and that if one wants to preserve leg room by not bringing anything other than a small bag and put that in the bin, they are entitled to. A small (and IMO totally incorrect) minority view holds that small items should be placed under seats so that others can use the overhead bin for larger items.

The nuance here is that this is a pax to pax request, when in the past this has been the request of an FA who is trying to make room and save time. This, IMO, is a critical difference. While I don't think any pax should be asked to give up their legroom if they only brought one item on board, a pax making this request of another pax is just not appropriate.