Agreed. The truth is that any technique can be countered if it is not performed perfectly (which means perfectly set-up, perfect timing, perfect accuracy, and perfect speed) and any technique (within reason, that no touch "chi ball" crap doesn't count) can be undefendable if it is performed perfectly.
The advantages that low kicks have over high kicks are:
Weapon travels a shorter distance, therefore opponent has less time to react
Less alteration in balance means both quicker recovery and also less likelihood of winding up on your head should things go wrong
Destroys the opponent's base of support and power (their legs) the decreasing mobility and stability
Less loss of balance means more powerful round kicks.
The advantages that high kicks have over low kicks are:
They attack more vital targets that can shut down the whole system (Brain, liver, bladder, heart, lungs)
In the case of straight line kicks like side kicks and push kicks to the body have maximal reach and can create not only a powerful kick but also a "posting/poling/wedging" effect to create or maintain distance
Both types of kicks are useful and effective if you utilize them correctly and both can be countered. So the more important question isn't the efficacy of the techniques themselves but how you are training said techniques and whether you can apply them effectively against resistance. Sadly, most TKD schools fail to do this; that (along with 10 year old black belts) is why TKD gets a such a bad name among combative oriented Martial Artists.