We can value whatever, but eventually there won't be anyone left to do the valuing. Impermanence doesn't stop us from valuing, but impermanence does guarantee that value won't last. Which may or may not bother us now, depending on how existentially grumpy we feel in the moment. But luckily, those feelings are impermanent as well.
All true, but of course the point of the idea we're discussing here is that there may not be any reason to assign special weight to the value of things in the present or the future, over the value of things in the past.