You could do that, or you could kill faster and keep the chain bonus alive.

Yes people could slowly kill a monster, but they would receive approximatively the same SP as if you fought at a normal pace.
It wouldn't be the pace of the battle that gave you an increase in SP, rather it would be fighting more difficult battles that would result in the most significant SP increase.
What does this mean in practice? You aren't giving any specifics. Also with so many party setups there is no way they can balance the system universally. This suggestion simply doesn't have any practicality. It would be filled with exceptions and would take years to balance correctly. You don't build a game off ideals.