Unless the GOP backs away from its opposition to racial quotas, it’s unlikely to break out of the teens. But if it stopped opposing them, a bigger chunk of the white vote might balk. It’s a little like Bud Lite marketing to cross dressers hoping for some incremental new customers, only to lose many of its core clientele.
The point being made is that black votes for the Democrats are very rational. If racial quotas go away, a huge chunk of the black middle class goes with it. That is why it will be a struggle for the GOP to get even 15% of the black vote. If employers hired and promoted based strictly on ability, blacks would tend to pool at entry level jobs. It’s not pretty, but there it is.
Trump getting the Black Vote in the high teens is very possible and would still be a catastrophe for Democrats. The models for how states and congressional districts vote changes if you do not consider the Obama years of 95+% Dem vote to be the future base line. To a large extent, what we are seeing is a reversion to mean. Think about What those models of congress look like if Republicans can get 15-18% of the black vote like they did when Reagan or Bush was president.