benignuman- I heard once that originally that is what the plan was. To go away for just 3 days. Afterall the shibud mitzrayim was supposed to be for 400 years. Once they leave for 3 days and receive the torah, they can go back to mitzrayim, EVEN IN THE NUN SHA’AREI TUMAH, because now with their new-found Gift, the torah, they have the greatest spiritual tool to rise above mitzrayim’s immorality . The power of the torah will protect them from forever getting lost, no matter how deep they sink, and how low they fall, even if they reach the 50th shaar of tumah.
Once Pharoah didn’t even agree to that, Hashem had no choice but to free the yidden, because they were in the 49th level of tumah, and, because they had no torah, were in danger of being eradicated forever once they reach the 50th.