Doesn't make sense even with your explanation. Sell a lv 90, Blizzard gains money and you save time. Terminating a lv 90 does nothing for Blizzard short of creating work.
"It’s not the job of the artist to give the audience what the audience wants. If the audience knew what they needed, then they wouldn’t be the audience. They would be the artists. It is the job of artists to give the audience what they need."
— Alan Moore
If you don't like your 8-9 90s you've leveled, find a way to like them. Try different specs, PvP, PvE, whatever works. Otherwise, they'll just sit there and become bank toons/gatherer toons.
You can delete a toon your self. This feature does not bring anything new to the table.
That is one thing I enjoy about EVE, farming and trading in-game to buy game time
Blizzard could not possibly make it work, look at what the AH/GAH did to Diablo III.