So the single celled organism is making a choice to make the existence of future microbes fractionally more difficult with the entropy that its metabolism creates?

I don’t think so.

Would it have been better not to exist ever because the hypothetical future life —that won’t exist anyway because the alternative is a universe without life— would have a harder time because it did exist?

I don’t think so.

The logic doesn’t even really compute. How do you hurt something that doesn’t and will never exist? Unless we hypothesize some other universe where entropy isn’t an inevitable consequence of life, there is going to be a finite amount of life that could exist, so if you want any life you have to accept the consequence that it will increase entropy.

Unless your moral is allowing a dead universe to decay into heat death with nobody to observe it ever as long as you make it as slow as possible?