When did the Chachamim pass the gezeirah on tumas haamim? If it was to keep the Jews in Eretz Yisrael wasn't it known that we'd be going to galus and living a good part of our national existence in galus? Even after the 2nd Bet Hamikdash was built it seems that most Jews were not interested in coming back and preferred to stay in Bavel? The same is even true nowadays. If that's so it didn't seem like much of a deterent.
The Gemara in Shabbos (15a) cites two Beraisos, one stating that the Gezeirah was enacted 80 years before the Churban, the other stating that Yosi ben Yoezer Ish Tzreidah and Yosi ben Yochanan, who lived several generations earlier, enacted this Gezeirah. The Gemara reconciles this by explaining that the Gezeirah was enacted in two parts.
The way I have always understood the Beraisa, which lists many Gezeiros that the Rabanan enacted involving Tum'ah starting from 180 years before the Churban when the Romans took control of the region, is that the Beraisa is saying that since the Churban was impending, safeguards were necessary to insure that the standards of Taharah were upheld in the face of the overwhelming feeling of impending doom. Since the people felt that the Beis ha'Mikdash was going to be destroyed anyway, there was a fear that they would stop upholding the proper level of Taharah, and therefore it was necessary to make safeguards. (This is my own theory.)
Dov Zupnik