Nirvana in This Life
Granted there's no persistent identity one can label "self," and granted that there is no continuity of experience beyond death, the importance of narratives about becoming other things, contributing energies toward other lives, or being something else is as a *metaphor* for this life's experiences.
The best referent for nirvana is related to craving, clinging, and anguish. What is terminated is only a dysfunctional connection to the world.
A "next life" is the next transformation of consciousness in THIS life. We should care about this because experiential quality is real and changeable; decisions today about what we shall do in the present affect how we perceive and engage the world in subsequent present moments.
It is pointless to try to stop existence, to go anywhere other than to the present, to the now, because there is no persistent identity which may "go" anywhere, except to return to oneself and improve awareness.
There is nothing which is eternal (endless, outside of time); samsara is simply experience and existence without nirvana's arrival. The importance of sentient beings is that they, like us, experience samsara, and that a number of us are dedicated to moving beyond this condition out of compassion, bringing about experience without anguish, as this is optimizing. Insofar as these names (such as 'buddhas') have meaning, they merely help us to focus our attention on actions and principles of value.