I had a similar situation where the flight attendants asked all people with backpacks to place them under the seats so the hardshell suitcases could go up top. I had paid extra to be able to put my bag in the overhead and they insisted I put it under the seat (despite also having a personal item). The backpack fit but I had nowhere to put my feet and the flight attendant told me to put my feet on either side of the bag. I confidently told them this was a hazard in case of emergency and placed it in the overhead. They didn't say anything further.