It’s really not hard. The best examples of cards with the earlier certs will be some of the cleanest ones ever graded because they spent less time in packs, pack-fresh copies were easily available and people had to be a bit more particular about what they are grading.
On the other hand PSA treated Pokémon like they probably treat the Sabrina the Teenage Witch TCG today. The stakes were lower and the level of experience of everyone involved in the process was lower.
So the variance in a grade is probably higher to begin with.
Then add into that the fact that strong copies stay in collections, weak copies are replaced and flipped, stronger copies are regraded/crossgraded.
The benefit of buying a freshly graded copy is that you are getting an unbiased sample of the distribution, whereas older copies will skew towards weaker copies whether its 2xxx, 4xxx, 6xxx or one day 8xxx. Because an older slab is something someone chose to sell for some reason and likely not something graded by the seller for the intention to turn around and flip.
There will always be a range in quality for any cert number. To make a sweeping claim that all 2xxx or 4xxx (which used to be a strong cert lol) cards are worse than 8xxx, you have to assume that somehow people have systematic access to a degree of quality today that didn’t exist when vintage booster boxes were a few hundred dollars. If you acknowledge that the top 5% quality cards submitted in 2014 are generally comparable to the top 5% quality cards submitted today, then logically the top 5% of 2xxx certs are going to be comparable to the top 5% of 8xxx certs.
As always, if you care about condition look at the card. If you are blindly buying a massive amount of cards the cert might matter to improve your average. But if you’re hand-picking and inspecting each card you buy then you will do better by inspecting the card than the barcode.