We want to live forever. What does that mean? First suggestion: I want it to be the case that at every future time I exist. But that's not enough. For suppose that time itself will come to an end in five minutes, and I will live for the next five minutes. Then at every future time I will exist, but my desire to live forever will surely not be satisfied by living for five minutes.
Second suggestion: I want it to be the case that at every future time I exist, and that the future be infinite. But that may be too much. My desire to live forever could, perhaps, be satisfied by the following scenario: I live in a universe with infinitely many very, very widely separated inhabitable stellar systems. After a year, I enter into a spacetime machine that can move me to an arbitrary point in spacetime, and this transports me back in time exactly one year (relative to some single reference frame) and moves me to another inhabitable planet, far from anywhere that I've lived before. I live there for a year, and this repeats. If t is a time two years from now, I do not exist at t on this scenario. However, I do live forever in subjective time--I have an infinitely long life, though it is all wound in on itself within a single year. Granted, it's a nuisance to have to get to know a new place every year, even though I do get to keep memories from the other places I've been. But this nuisance has nothing to do with the satisfaction of my desire to live forever. But I could live forever in the ordinary way, and have that nuisance happen to me--I could be every year transported to another planet, far away from where I have ever been before, but not transported back in time.
I want to suggest that it doesn't really matter whether I'm transported back in time every year or not. Third suggestion: What we want is an infinite future life, regardless of how that future life is arranged in spacetime. On this suggestion, it is subjective or personal time that matters vis-a-vis living forever. I think much the same is true with respect to many other temporally charged ethical matters--when only one individual matters, it is her personal time that matters, and when there are multiple individuals mattering, it is their group interpersonal time that matters (imagine a group of individuals all zigzagging back and forth across spacetime).
And if we have the idea that we should take as ontologically more basic what is more significant to us as persons (taking ethics as first philosophy, or something like that), then we will adopt personal time as the focal or primary sense of temporality. If we do this, we will be B-theorists, because we will have little need or room in our ontology for an objective impersonal moving present. I suspect this approach is faithful to both Einstein's notion of the relativity of time and to the basic insight in the Aristotelian idea of time as the measure of change (just take time to be the measure of change within each given substance, but do not insist on embedding these changes within a single temporality).
A related issue is discussed here.