At the latest SEA-CCC meeting, there was a discussion about the really noticeable visual defects in the new C64C cases folks can buy brand new from iComp, previously Pixel Wizard and before that Kickstarter. They all use(d) the same moulds. My SX-64-styled case is particularly bad, which from what I've heard is very common with this particular product.
However, I remember looking at one of my spare 64C cases and seeing streaking there, too. It just wasn't as eye-popping possibly due to the color, or possibly the chemical composition of the plastic used at the time. Back in the day, the plastics could yellow over time whereas today the plastics used generally don't, etc.
In any case (*cough*) here are some examples of streaking on an original case as they shipped from C=.
As you can see, there is definitely streaking in the original 64C case. It's not nearly as prominent, but it was indeed there.
One club member and local C64 legend (kentsu, of Dr Evil Labs fame) posited that there could be a mistaken assumption that the original cases were perfect, but the fact may be when we were kids we may not have been looking at the products under close scrutiny.
Either way, it is interesting how we sometimes look back towards the past with rose colored glasses. But it's also true that the streaks in 2024 do seem more prominent.