That thousands of alterations have been identified, and are continuing to be identified, by collectors hundreds of miles away from the physical cards says a lot. They've identified ways to identify alterations that are not being used by grading companies. Their methods include looking at images, and provenance. These methods will continue to be used, whether or not grading cards use them and, no doubt, sometimes try to hide provenance.
Thus, grading is done not just by the grading companies but by the hobby. Hive mind, some might call it. If graders can't or won't identify alterations, others will be doing it for them publicly.
Of course, part of it is that buyers have to quit being dumbasses, and use some old fashioned common sense, which might be a far bridge to cross
