If by introducing demographics, you are referring to age: one can - and should, for various purposes - partition the population by age. Doing so is somewhat more arbitrary than, say, by vaccination status, but both can be clearly well-defined. The difference is that no-one chooses to get older, and so there are no ethical issues with it.

As for the argument that everyone will get it eventually: For one thing, the pace matters, the recent shortage of ICU beds is a negative public-health outcome, and the statistics clearly show the benefit of vaccination in this regard (few health issues have better statistical evidence.) This alone shows that there is a reasonable basis for so dividing the population.