The more I read about Chanukah, the less I understand why we celebrate the holiday. The victory, as I understand it, was costly and very short-lived. Yes, the Temple was restored, but only to be destroyed ~232 years later, perhaps as the result of nationalistic tendencies that developed during the Hasmonean period. So Chanukah to me seems like the beginning of the end of the Second Temple era. It's fun but I struggle to enjoy it for that reason. What am I missing?
A Chanukah Shiur by Rabbi Joseph B. Soloveitchik, delivered at Moriah Synagogue, circa-1950, transcribed by Rabbi Nisson E. Shulman, explains the meaning of Chanukah and shows that Chanukah itself is basically a "hidden" celebration. We speak of the fight against the Syrian-Greeks, but that was not the main battle. As with many of the trials, tribulations and struggles of our people, the main fight was against our brethren. While the "outside" political victory was short lived (and betrayed by the Hasmoneans themselves), the real victory was permanent and continues today as we celebrate bris milah, shabbos, and rosh chodesh.
The problem of Chanukah was not destruction, but defilement, tum'a; tim'u kol hashemanim [=they defiled all the oil (a quotation from Maoz Tzur)]. Kesheamad malchut Yavan? Lohi, ela kesheamdu hamityavnim... [="When the Greek kingdom rose" (a quotation from liturgy)? No, when the mityavnim, Hellenists, rose]. But our rabbis sought to hide this as much as possible because dibru belashon nekiya [=they spoke with clean language].
Alma deisgalya [=the exposed world] was the war against the Greeks. Alma deiskasya [=the hidden world] was the war against the Hellenists.
So why don't we see this in the texts? We do, but we must lift the cover, since our sages did not want to speak evil about the Jews, even about the wicked ones, and so they hid the real story between the lines.
The whole story of Chanukah was euphemised. This is particularly apparent from the prayer, Biyemey Matityanu... The excessive repetition expresses it very well. "You, in Your great mercy, stood by them in the time of their distress; ravta et rivam - you fought their battle". More explicitly, "danta et dinam - you judged their case." Do you want us to be even more explicit? "You delivered the strong into the hands of the weak". Even more explicitly? "The many into the hands of the few". Still not satisfied? "The pure into the hands of the impure". Do you, then, demand an even more explicit statement?" Very well, but we say it sadly and very reluctantly, reshaim beyad tzadikim, vezeidim beyad oskei Toratecha" [="the wicked into the hands of the righteous and the sinners into the hand of those who involve themselves with Your Torah"].
From the time the Jews re-entered the Heychal [=main room of the Temple], the war against the mityavnim was virtually won, and mityavnim were nevermore in the majority. So this is an eternal miracle. That is why we still celebrate it....
UPDATE Note the rashi in Beshalach about "chamushim" points out that 80% of the Bnai Yisrael died before the Exodus to enable it to occur. This was about a millenium before the Chashmonaim so we see that the problems can recur. However, since the days of the misyavnim, we have not been in as perilous a situation (though it may be since we are in galus and they have never had such political power again). Even in the days of the "Enlightenment" and the attempts by the "secular" to destroy Judaism, they have not been in a position to force the destruction of Judaism (even seeing what has happened on the modern college campuses).
I gave a Shi'ur last Friday night about "Interesting Hava Aminas Relating to Ḥanukkah", and the fist Hava Amina I discussed was the possibility that we should not celebrate Ḥanukkah at all.
This is a very famous question, but not for the main reason you gave. First, the timeline provided in the question needs to be challenged. The Ḥanukkah revolt began in approximately 165 BCE (depending on interpretation), and the "kingdom" of the Ḥashmonaim was established sometime thereafter (also depending on interpretation), but not later than 140 BCE. However you calculate, the Temple was destroyed no earlier than 68 CE. In other words, even taking the shortest possible number of years from the establishment of the Hasmonean dynasty to the destruction of the Temple, the timeline spans over 200 years (see Maimonides, Laws of Megillah and Ḥanukkah, Ch. 3). Even taking your timeline (which I don't begin to understand), 150 years is not very short. Many "empires" lasted for far shorter periods (see Wikipedia list of empires here).
Regardless of all of the above, we aren't celebrating an empire or any particular dynasty. We aren't even celebrating any particular military victory in and of itself. To oversimplify, what we are celebrating is Jewish religious continuity, which was enabled by the military victory and establishment of the Hasmonean dynasty. Please see Peninei Halachah, Hil. Zemanim, Ch. 11 on Ḥanukkah, in which he discusses at length the reasons why we celebrate it even today, despite the many reasons not to celebrate it (including some of the issues you raised - though not the timeline).