Almost never for the injury itself: a torn cruciate is painful and disabling but it is treatable, and even without surgery many dogs reach a livable result with strict management. Euthanasia enters the honest conversation only when a cruciate rupture lands on top of other serious problems, severe arthritis everywhere, major illness, extreme age, or when every treatment pathway is genuinely beyond a family's means and rehoming is not realistic.
If the money is the blocker, read the options before deciding: lateral suture surgery in the right dog costs a fraction of TPLO, structured conservative management costs mostly discipline, and payment plans and charity clinics exist. A $250 independent opinion on which cheaper pathway is legitimate for your dog is a very small spend against this decision.
If you are at this page because a vet has genuinely recommended euthanasia over a knee injury alone, in an otherwise healthy dog, get a second opinion before you act on it. That recommendation is unusual enough to deserve one.
Send the case to the surgeon. A written independent opinion, from $250.
Ask the surgeon. A written independent opinion, from $250.