I dont really get it. Of course an old cable will only support the standard that was around when it was produced. So if you have a port that supports a higher bandwidth than the cable then obviously the throughput will drop down to the level of the cable.
As long as its downwards compatible i dont see the problem. You can plug your old USBC cable into your brand new laptop and it will work just fine as it always has. Do these people just expect magic?