no we are just doomed no matter what

It may not have any effect here. The ~20% minority journalists may just focus on minority topics enough to offset the roughly 30 percentage point difference.

But generally, the motivation behind this field of research is that if you never see a black doctor on TV, you may not think becoming a doctor when you’re black is something you can strive for. Nor might your parents or teachers or future potential employers.
Which ultimately cuts of career paths for a ton of people and therefore damages those people as well as society as a whole.

And well, it’s largely secured that if most of the media is not a minority, then it’s a lot less likely for them to hire a minority actor as the doctor. So, how the minority representation is in the media, is a relatively important figure for this field of research.