I've been thinking about the subject of this post
for a at least a couple days. And finally, I decided to write down some indisputable truths about the end of Buffy Season Seven (and beginning of AtS5). Not all of these are self-evident, but you will find on careful reflection that no one could honestly disagree in good faith with any of them.
1. Even if Spike hadn't had that amulet and even if only one slayer had managed to survive Buffy's original idiotic plan, the humans could still have won the battle just by retreating to the hellmouth and cutting down the Turok-Han one by one as they came out. That goes double if they'd blow a hole in the roof so they only needed to guard at night (about 8 hours a day in Southern California in May).
2. The closing of the hellmouth was performed by men without feminine help. Spike didn't know what the amulet did when he wore it. Buffy didn't know when she gave it to him and neither did Angel nor Lilah when they passed it on. The heroes of closing the hellmouth were the Wolf, the Ram, and the Hart who made the amulet. All men. Unless the wolf was a she-wolf, in which case it was a one-third feminine effort.
3. Angel was supposed to wear the amulet. Why would the Wolf, Ram, and Hart want Spike around keeping Angel honest? They were trying to trap Angel inside the WRH organization so they could corrupt him more easily. Incorporeal but with the WRH staff to do his bidding in the world he would be entirely at their mercy and quickly go bad.
4. Those slayers were going down inside the hellmouth. The went in with about a couple dozen and came out with Kennedy, Buffy, Faith, Rona, Violet, and maybe a couple others. Even if the battle had turned their way by the end, and there's no evidence for that beyond the music, there weren't enough left to survive even the occasional very good day for the thousands of remaining demons.
5. The thing that made the original slayer spell awful was the death sentence. Slayers die young. There's no retirement plan. Willow's spell wasn't a curse because it doesn't have that limitation, so far. We all have to do things we don't like in life like working for a living, or homework, or fighting a war, or being infused with the spirit power of an evil demon so we can fight the forces of darkness. The difference is the surety of death.