I feel that we drink the 4 cups of wine on Passover because of the 4 things G-d gave the Jews as they left Egypt. One of those things was Individual Freedom. Can a people, such as us Jews, live in a foreign land and celebrate Freedom as a Nation, A People? I do not think so. G-d gave Israel to the Jews to practice Freedom, not only individually, by as a People or Nation. If this the case, the fifth cup of wine should be for the land of Israel?
The Jerusalem Talmud actually holds that we should drink a fifth cup. But we follow the ruling of the Babylonian Talmud that only four cups are drunk and the fifth cup is poured, but not drunk. This cup signifies the fifth promise that G-d made to the Jewish People when He took us out of Egypt: that He would bring us to the Land of Israel. This last promise has not yet been totally fulfilled , for until Moshiach (the Messiah) comes, we will not be totally secure in Israel (notice all those nations surrounding us, still out to destroy us).Thus, we pour the fifth cup, but do not drink it. When Eliyahu HaNavi returns to announce the arrival of Moshiach, then we will drink it, for then the last promise will be fulfilled. Thus we call it Eliyahu’s cup.
The Jewish People certainly can celebrate Pesach (Passover) outside of Israel. The very fact that so many Jews do celebrate it and do so many other mitzvahs in the exile proves it is very possible. What you must mean is a point the Talmud itself makes in Tractate Berachos: as long as we are in exile, it is much more challenging to keep the mitzvahs than it will be in Israel after Moshiach comes. The influence of and pressure to conform to foreign cultures weighs heavily upon us. Here you are certainly correct. But don’t forget, from our challenges we grow the most and accomplish the most.