Why is downshifting to first bad?

Pretty much title. Everyone says it’s bad but there’s never a concrete explanation other than “it puts wear on ur transmission”. If you second, clutch in, neutral, clutch out, rev, clutch in, first, the car goes into first like butter. I see no reason why this would be bad for the car.

As a benefit, because it’s so long you can engine brake much more effectively in first than second.

So, assuming no money shifting, proper double clutch rev matching, why is shifting into first at 20mph bad? It’s the same as any other gear just longer.

Edit: it seems like it’s not bad as long as you double clutch properly. My car has a 38mph red line in first which seems abnormally high compared to others and I think that makes some confusion about the usefulness. I’m not over revving my car coming to a stop in first at 20mph and in second it idles at like 12mph.