[Eric72]

In my opinion, the sale/trade of doctored cards is an issue in both the modern and vintage markets. Third party grading companies have (unfortunately) encapsulated many of these altered examples. The number of doctored cards with numerical TPG grades is much more likely to increase over time.

By way of example, consider a card trimmed along one edge. The card being in a "slab" would severely hinder one's ability to examine the edges and look for relative oxidation, striation patterns, etc. Looking only at the front & back would be an incomplete, insufficient way to inspect the card.

Since this hypothetical example would likely sell for more money in its TPG holder, there is little incentive for the average collector/investor/flipper to remove it. Chances are, it would remain "slabbed" and the card doctor card butcher would have "gotten away with" another alteration.

This is, of course, just one of innumerable possible scenarios. My main point (to answer one of your questions) is that this is not confined to just modern or just vintage. It's not even limited to just sports cards. The problem is pervasive and pernicious.