Let's reach the stars first and worry later about how many zillions of years of fun remain. If we eventually run out, then we can abort or wirehead. For now, it seems like the expected awesome of creating an intergalactic posthuman civilization is pretty high.

Even if we create unimaginably many posthumans having unimaginable posthuman fun, and then get into some bitter resource struggles as we approach the heat death of the universe, I think it will have been worth it.