That’s NOT what they said. They don’t want to sign an unconditional armistice with a political entity that has shown the inability to not revert to committing atrocities.
We don’t know what their solution would be because they haven’t given one, but there are several possibilities that don’t involve fighting the Horde until they’re all dead:
- Politically and militarily dismantling the Horde and occupying its territories.
- Internment or concentration camps (a la after the Second War) until complacent individuals can be integrated into Alliance society.
- Deporting the orcs and Forsaken via the Dark Portal. Alternatively, using Draenei space ship technology to send them to Argus.
Anduin’s concerns about creating resentment leading to future war are not unwarranted, but Tyrande and Grenn are not wrong about what will happen if the Horde is allowed to continue. History has borne that out repeatedly.
Yes, the above solutions could involve more fighting to employ, but the incoming Horde leaders appear to be willing to make peace at any cost. Before Saurfang exposed Sylvanas, they were in fact prepared to march on Ogrimmar with the Alliance military and kill as many of their own people as necessary – tens of thousands of them even – to force peace.
Nothing in the current characterization of Baine, Thrall, or Calia suggests that they would stand up to the Alliance if they imposed any kind of conditions on the Horde. Lor’themar would presumably agree to anything short of the Blood Elves giving up the Sunwell.
EDIT: The Alliance should not miss the opportunity to take advantage of a pro-Alliance Horde leadership.
Germany was politically dismantled, split into two, and occupied for a decade after World War II in part because it couldn’t be trusted to not remilitarize. German leaders were arrested, tried for war crimes, and executed. Japan was occupied, had a democratic government forced on it by American high command, and was forbidden from having a military force capable of waging war (the famous Article 9 of the post-war constitution).
These were not merciful responses, and they had some extremely negative impacts on the self-image of those countries (but also some economic ones in the case of West Germany and Japan that were extremely positive), but they were practical responses in light of the death and destruction Germany and Japan instigated. Being practical doesn’t make those responses evil.