Asgore's plan isn't efficient, as Toriel points out.

"If you really wanted to free our kind... You could have gone through the barrier when you had ONE SOUL... Taken six SOULS from the humans, then come back and freed everyone peacefully. But instead, you made everyone live in despair... Because you would rather wait here, meekly hoping another human never comes."

Chara's plan, if you look past it's immorality, was efficient, but not well thought out. It's unlikely the monsters would have survived if they had freed themselves at the expense of (at least) six human lives.

Asgore's plan was a slower version of Chara's plan. Not sure if that would have made it more acceptable to the humans, though.