I agree this would be a reasonable solution to the problem..

but good luck finding a veterinary clinic that will destroy perfectly healthy animals. I have called (on occasion) and am turned away EVERY time..they want you to spay/neuter and release (which is what I have ended up doing every time).
And getting rates much cheaper than a client would pay...again, good luck with that.
My rates keep getting raised year after year. Yes, I get a better deal on my spay/neuters than most, but keep in mind that a thousand of my hard earned dollars only alters about ten cats!! Doesn't feel like much of a difference is made in the long run...but I put my head down, and keep plugging away at it, none the less.

Many municipalities do NOT pay their local animal control authorities to deal with unwanted cats (feral or otherwise), so many taxpaying residents are turned away when they try to solve a problem on their own.....(Ladysmith, for example does not have a contract in place for abandoned/feral cats) This is another issue that should be brought forth in this debate!
If the municipalities were held accountable, and made funds available for more humane euthansia methods,(through taxation) the incentive to the animal control authorities to move in this new direction would be greatly increased!!