0. The simple meaning of this statement is that the Egyptian empire would never have ended and we ould still be slaves there. This is hard to believe. Would history really have stood still? Wouldn't Egypt eventually have ceased to be an empire? Empires come and go. So had G-d not taken us out, we would be free in some other way. The tower of Babel teaches us that every empire will eventually come to internal fracture and crumble away, even if no outside force destroys it. This was implanted into the nature of empires by G-d at that juncture. So Egypt would surley have ended at some time.
1. On the simplest level this statement should be understood as a statement of out mental attitude. We should try to imagine that we might still be slaves in Egypt. And then imagine that our children and grandchildren would be there too. And from thinking of how terrible this all is and then realizing that G-d brought us out, we are the more grateful to Him and will praise Him with real feeling and conviction. So this is a mental exercise using the imagination. It is an duty for us to go through these steps in our imagination so we can praise G-d truly.
2. On another level it should be understood as a statement of a certain transformation which occured in the Jewish people that made us perminently free people. Being "meshuabad" means to be subserviant to another people. Most ancient people have died out over the years. Where are the Visigoths? The Huns? The Medes? They were either wiped out or absorbed into another people. The Jews have suffered the same circumstances as these people, yet we have not died out. We have suffer persecution and subjugation. We have faced assimilation, but where other nations have disolved in these circumstances, we have survived. In part this is because we have identity re-inforcing holidays like Passover to remind us of who we are. But also there is a miraculous character to our survival. G-d has caused this. By taking us out of Egypt He made us a free people. So we can never truly by subjugated. No matter what we suffer we will remain inwardly free. Anything that G-d does is permanent, so our freedom is permanent since he took us out of egypte, (He and no angel). The third temple will be permanent becasue He will build it. This is the meaning of the statement that had G-d not taken us out of Egypt we would still be slaves in Egypt. The meaning is that we would have eventually fallen prey to some "shiabud" and either be wiped out or assimilated into another nation. So we would today be slaves to some forein culture and not even realize we were truly Jews. We would be slaves to some "Egypt" and we would think we were free. True freedom is being who you really are. Expressing your soul's desires. That would have been lost to us through history's inevitable pressures. But G-d took us out and made us free today. WHo is free? He who Keeps the Torah. So we can thank G-d today that we are able to be here now celebrating Passover and not part of some other nation.