Imperfect self-sacrifice is never lost

טעם שזכה לוט להיות עם אברהם אע"ה וגם הוא גופא שזכה לצאת ממנו דוד ומשיח נראה כי הי' בזכות הרן אביו אף שלא הי' מס"נ בשלימות ולכך נשרף.

The reason Lot merited to be together with Avraham Avinu, and even Lot himself merited that Dovid and Moshiach would descend from him, appears to have been in the merit of his father Haran - even though Haran's mesiras nefesh (self-sacrifice) was not complete, and therefore he was burned.

The Sefas Emes asks why Lot was zocheh to such great things. He explains it was in the merit of Haran, who was willing to give up his life for Hashem like his brother Avraham, even though his readiness was imperfect - which is why he did not survive the fiery furnace.

אעפ"כ שריפתו הי' בכלל קידוש השם ונשרף בעבור השי"ת ואין הקב"ה מקפח שכר כל ברי' לכן זכה לכל זה:

Even so, his being burned was included in kiddush Hashem (sanctifying Hashem's Name), and he was burned for the sake of Hashem Yisborach; and since the Holy One does not withhold the reward of any creature, therefore Lot merited all of this.

Though Haran's sacrifice was flawed and he perished, it was still a death al kiddush Hashem. Hashem never lets even a partial act of devotion go unrewarded, so that incomplete mesiras nefesh became the source of Dovid Hamelech and ultimately Moshiach.

Summary: Even an imperfect act of self-sacrifice for Hashem is never lost - Haran's flawed mesiras nefesh earned his descendant Lot a share in Avraham's world and a line leading to Dovid and Moshiach, because Hashem withholds reward from no one.