Throughout the parshiyos of Shmos and VaEra, Moshe asks Pharaoh to let the Jews go for three days to worship God. Well, as history attests, Pharaoh refused to let us go - indeed, God Himself assured Moshe that Pharaoh would refuse.
Nonetheless, suppose Pharaoh had said "You know, you're right. I bet if they have a break, they could come back and work much better later. Go ahead, take them out, see you back here in a few days."
What then? Would the Jews have come back?
(For all those who have been complaining that my posts have been too long -- how's that? :) )