It would be "jumping the shark" narratively speaking. Why did Jehovah store "Heaven's least wanted" in the same gravity well as His perfect creation? It would have spoiled the conflict points later in the story. It is the Gilligan's Island paradox.

According to the Bible it was god who "hardened" Pharaohs heart anyway. So if he could harden Pharaohs heart why could he not soften it, or did he just want to show off how effectively targeted his weapons of mass destruction could be ?

Interestingly, some people have proposed that the killing of the first born may have been down to a common fungal infection of Ergotism, which infected the grain making it poisonous. It being said that in times of famine, (and the others plagues could have caused famine,) the Egyptians had a tradition of only feeding the first born from the granaries, the other children and animals had to fend for themselves. This meant that the damp infected grain was eaten only by the Egyptian first born, while the Hebrews, who did not have that tradition, were spared, or at least only suffered mild collective poisoning.