What would have happened if the pharaoh hadn't let the Israelites leave Egypt?

The nation of Israel would have continued as slaves, possibly assimilating completely, and would not have experienced a miraculous deliverance from Egyptian bondage.

The Ipuwer papyrus describes Egypt's experiencing the Plagues: "Pestilence is throughout the land....the river is blood, death is not scarce...there is no food...neither fruit nor herbs can be found...barley has perished...all is ruin...the statues are burned" (Professor John van Seters, Journal of Egyptian Archaeology no. 50). The plagues were also described by ancient historians, including Herodotus and Diodorus. The Exodus is mentioned by Strabo, Berosus, Artapanus, Numenius, Justin, and Tacitus.

But in any case, few nations are content to record embarrassing setbacks honestly. Even today, British and American textbooks describe the American Revolution in very different ways.

An example of the above principle:

The destruction of Sennacherib's army at the walls of Jerusalem was denied by secular theorists, because the Assyrians made no mention of it. But then it was found that Berosus and Herodotus both state that Sennacherib's military campaign in Judea ended in plague and defeat. It should not surprise us that the Assyrians themselves didn't record their own losses.

It is only the Hebrew Bible, because of its Divine origin, that exposes the faults of its own people and even magnifies them.

In no other religious text can one find such openness. None of the Israelites were immune to strong criticism: Abraham (Genesis 16:5), Reuben (Gen.ch.35), Simeon and Levi (Gen.ch.34 and 49), Judah (Gen.ch.38), Joseph's brothers (Gen.ch.37), Moses (Numbers ch.20), Aaron (Exodus 32:2-4), Samson (Judges 14:1-3), Eli's sons (1 Samuel 2:12), Samuel's sons (1 Samuel 8:1-3), Saul (1 Samuel ch.15), David (2 Samuel ch.11-12), Solomon (1 Kings ch.11), and many others.