If slashing is so ineffective, Persian Shamshir would be the ultimate example of a useless weapon. The weapon most similar to the Roman Gladius (Caucasian Kindjal) was used mainly for slashing.
Also, whether Roman legionnaires occasionally used Gladius for slashing is not relevant: in the heat of the battle they probably smashed their enemies with sword handles
The point is that Gladius was
constructed as a a double-edged sword. Thus, the slashing function was "built in" from the outset. With that in mind , any argument in favor of it's purely-stabbing function is plain silly.
Smallsword is a pure stabber, Omani Kattara (with a rounded point) is a pure slasher. Gladius was
designed to do both.