Yep, agreed.Cat 6 and 6a were created to address specific advancements, namely increased bandwidth (10Gb) and all the things that need to happen to use that bandwidth reliably. If the system is working no amount of cable swapping is going to make any difference in regards to ethernet frames being delivered across the wire, i.e. using 6a for a 100/1000Mb link will offer no improvement.
The cost difference between, say, Cat5 and Cat6 is the extra wire. The strands of an Ethernet cable are twisted more tightly in the better grades of cables, which results in the higher bandwidth (due to common mode rejection). Shielding can help even more. I don't need gigabit Ethernet in my home, but I still have it wired that way just for future-proofing.