Pharoh could have caught cold, had a fever: he could have died in Mitzrayim. They would have been busy with his funeral so they couldn’t pursue Yisrael. All kinds of things could have happened.
The miracle was entirely unnecessary for the purpose of saving them. But the miracle was done just for one purpose: vayaaminu b’Hashem — to give them the gift of emunah (faith). — Return To Hashem (#842)