To be honest I think it will go the other way and theming will be reduced to the point where a designer isn’t needed. Maybe even themes aren’t needed. But, where does this leave designers? If we’re not writing HTML and CSS what is there left for us to do?
On Twitter, the other day, I saw someone asking why he should learn React for use in WordPress. He hasn’t done so, and it seems people have called him lazy because of it. I think this is incredibly unfair. Not everyone has to learn it. Perhaps he has no need for it. Perhaps his clients are happy with the default blocks. Perhaps his workflow doesn’t call for it.
Whatever the case, it’s up to him: he knows what his role requires and perhaps React is not it. If he’s asking to be convinced that learning React is the right thing to do, then it sounds like he doesn’t need it at the moment.
There’s enough division in this world without introducing it into our job. This is about more than just CSS in JS. It’s about building the best possible thing in the best possible way.
Designers talk a lot about empathy. Considering the needs of the website users. I think more empathy and understanding is needed here. Front-end development is a team sport – and it needs people with varied skill sets to create the best product.