They wanted to respect his wishes but couldn't accept melting down the old brain tech. Could be valuable.
The teddy bear dissolved in the acid. Smelled awful.
Wara and Paul [j: I forget if I named the future people, maybe these are their names] cracked the teddy-bear head open and pulled the "brain" part out before they dissolved the body.
They felt they respected David's wishes but little did they know the "brain" was entirely self-contained and powered by solar and atmospheric renewable power!
They kept the "brain" as a reminder of their weird encounter with a guy from the past. Little did they know they made his existence infinitely more horrible by disconnecting him from the teddy-bear body and destroying it!
They lost interest soon after. David's brain—a permanently suffering ball of old technology, with no inputs or manner of communication—ended up on a messy sunlit shelf!
David's brain couldn't sleep and didn't need to recharge! David couldn't see, hear, smell, touch, or feel vibrations! He couldn't talk or contact anybody! He had no sense of where he was and how much time had passed! Yet his brain, the horrible solar-powered machine brain, continued thinking, thinking, thinking, no matter how badly he wanted it to stop! All he could do was think and wait! Those were his two options! Thinking and waiting! Thinking and waiting in a mysterious, sensation-devoid hell! Has it been an hour, a thousand hours, a century? David had no way of knowing or even guessing! He begged, mentally, to be put back into the teddy bear.... It was humiliating but at least he could see and hear! But no, he begged those kids to kill him, and they removed him from the beautiful teddy-bear body and trapped him in an insane hell impossible to escape from! And always... Always thinking, thinking, THINKING!!