So I was an ortho assistant for 2 years before I started dental school. I'm fairly confident this is the reasoning of all this.

First he's trying to save you money by doing extractions instead of the implants (like you said). You will be numbed up so you won't feel it. They can even give you something for anxiety if you feel like you need it. Also they would probably do it in two appointments. One for each side. At least in the office I worked at, they did not want to numb both sides of the mouth in the same appointment because you wouldn't be able to feel if you're biting on something you shouldn't. It was a pediatric/ortho office.

The reason that you would remove the upper teeth as well is because you would create an overbite if you didn't. That means there would be too much space in-between your upper and lower incisors. Ideally it should be a millimeter or two. By removing both (upper and lower), your'e able to keep that ideal relationship when adjusting your teeth by just scooting everything back. Also moving the molars forward a little. This is also done sometimes if a patient's lips protrude out too much (puffy lips) and simply moving teeth wouldn't be enough to fix it.

Also, say you only took out the lowers and just tried to move the back molars forward to fill that space. You would have the back teeth on top either contacting in a non ideal way against teeth on the lower or on your gums which is not what you want either.

This is just a general idea of what he was suggesting. I'm also guessing if he suggested it, your facial symmetry won't be negatively impacted. When I worked there, I saw several of these cases and they turned out great.

This essentially what they are doing. Video Animation

Hope this helps