Couldn't he have used Sacrificial Protection in the Astronomy Tower before being killed by Snape? Wouldn't that have saved countless lives in the future? He had already accepted his death when he said, "Severus, please."
Dumbledore was not in a position to sacrifice his life for someone else. The only life at stake was Dumbledore's own.
In contrast, Harry giving himself up in the Dark Forest works because he could have run or left, and didn't, to protect the people in the castle.
It could have protected them from Snape.
They weren't in danger because
Snape's a good guy.
Dumbledore's death was coming anyway, due to:
- a cursed ring that had burned his hand and poisoned him.
- the potion he took at the lake.
He was already dying. According to Snape's memories revealed in book 7, the whole scene was pre-arranged by Dumbledore and Snape in order to increase Snape's authority with Voldemort and other Death Eaters, and to spare Draco.