Idk I am definitely not as well versed in this as you are but I think a lot more factors go into play than just album sales. Like you said it's about demographics. But also radio plays, streams, concert tours, etc.
I am hearing less and less of Kendrick outside of debates for AOTY. That's all.
edit: Also I didn't just mean Future, was just using him as an example. Cole & Drake were two guys mentioned by Kendrick and they just have such a bigger following. Although to be fair Drake's following is something that is rarely seen in most genres because of his pop appeal.