This is a good topic! It involves a lot of aspects that extend beyond pokemon. I think what Jason said is the answer, “certain cards”. Gary also mentioned the Gold stars, which I think can go either way. The only thing that might save them are the amount printed, which no one knows the exact amount.
This discussion has been around since pokemon cards were first graded. There have always been different types of people grading cards. Ranging from strictly collectors, to the fleabayers (fast cash guys). The youtube collectors really started to pick up a few years ago. I am not big into youtube, but if it helps people get into the hobby I am all for it.
The best way to make an assessment of possible saturation is calculating how many people are actually collecting. What brought the hobby to this point today are the amount of actual collectors. It is difficult to imagine, but back in the day people just collected, and didn’t buy and sell. The more people you have collecting, the more consistent the market. The more people you have constantly buying and selling, the more saturated the market becomes because everything is constantly circulating.
If you look at any bubble burst, it is the temporary or false foundation that is the cause. With pokemon, and any hobby, a temporary boom is usually temporary because of the temporary components; the people in it for quick cash. If you see a ton of people only grading and selling, that is a warning sign.
This happens in sports cards as well. The patch/memorabilia cards which are the most valuable modern cards in the past 10 years have a similar dark cloud always looming. There are youtubers doing box breaks who can’t pronounce majority of the athletes names. This is not to say businesses should not exist, we need sellers, ones who know what they are doing. Not people out of their depth who do not have the proper funds, experience, knowledge or patience.
Basically, if you look at the housing crash here in the US, it was the result of people buying something they ultimately could not afford. This is similar with any temporary boom. People thinking they are going to make it big, but then realizing, oh crap, they were dreaming, and just like everything in life, it takes years of constant work and planning to make a successful business, a successful anything.
Ultimately, I think PSA is still getting its footing in Pokemon. As the amount of collectors grow, the less actual raw mint-gem copies exist on the market. I am not engaged in the youtube community, but if people are who visit here, take a look at how many people have an actual collection. Oh and if you are worried about your items saturating, as Gary already touched on: the older, the rarer, the minter, the better!