Rav Hirsch: Vaeira III
“Thereupon Moses answered Pharaoh, ‘Try to glorify yourself above me; for which time shall I entreat for you and for your servants and for your people that the frogs be destroyed from you and their houses?” (Exodus 8:5)
The motive for Moses making the exit of the plague itself into a sign and not simply undertaking Pharaoh’s request, lay in the request itself. Had Pharaoh been already convinced, he would simply have let the people go. But as it was, it was apparent that all he wanted was to be freed of the plague.
That was why the departure of the frogs was itself to be a further instructive sign; perhaps this would produse the desired result.
Accordingly, the frogs are not simply to go back from where they came, that could be taken as merely the cessation of God’s activity and a return to normal conditions.
They are to die, but as emphasized repeatedly, “Only in the river shall they remain,” not all of them that could again be taken as some phenomenon of nature.