10 lawns is not enough to buy a $10,000 mower.
Cart in front of the horse.
Selling a good running mower does not make sense when you will be left without a back up.
If you made enough money from this mower for it to have paid itself off then this mower does not own you anything and is another reason to keep it.
If this mower has not paid itself off then you should not be replacing it.