A major factor in bolstering the Forsaken's numbers is that risen humans are told they aren't welcome in the Alliance anymore by the Forsaken and Val'kyr. But what if that wasn't the case, and the Alliance picked up on this and told EVERYONE before sending them to battle and making sure its engraved into their thoughts and even written down on paper
"If you are killed in battle and risen into undeath you are STILL WELCOME HOME IN THE ALLIANCE SO LONG AS YOU DO NOT PLEDGE YOURSELF TO THE FORSAKEN, even if you go insane from the raising it is temporary and you will be granted clemency for your actions as acting under the influence of the enemy's magics and what you do in your first moments as undead will not count against you"
That way when the Forsaken pull the "Join or Die because no one else will take you" they can say "Nope, I got a card that says they will still accept me". Now obviously you'd have to be a registered soldier that could be able to prove who you are and not some thoroughly rotten Forsaken attempting to pose as a fresh Alliance soldier, maybe Priests and Paladins could determine truth or not.
And if risen Alliance continue to join the Forsaken after all this Varian could go to Tirion and say "Hey buddy, the Undead are forcing our troops into service in undeath even after we established that as undead they are still welcome home, you gota help us because they are binding undead"
This way Sylvanas is robbed of her greatest weapon and will have to resort to mass plaguing the lands to stave off invasion, which would only piss off the Cenarion Circle and Earthen Ring like nothing else.