Most of that thinking is due to the writings of David Lewis -Williams , which have been roundly refuted .
Rather than go into detail two problems (there are others but just in reference to the above ) are ; what hallucinogens produce only the very limited number and type of motifs found in rock art ? . If you have had no experience ask or read of others experience , it tends to be far more varied and interesting .We can't possibly know who engraved the prehistoric marking . Some shamen , or what is described as shamen by ethnographers (it can be a much wider category than the original meaning , encompassing almost anyone involved in religious /magical practices ) have been shown to have engraved motifs in the historical period and this has been extrapolated to periods and cultures that we can't possibly have evidence for .We also know that other obviously non shamanic types have also been shown to engrave rock art but this is conveniently ignored ,children or shepherds don't seem to have the same romantic cachet .