Originally, the term “cocktail” meant any distilled spirit (in the U.S., pre-Prohibition, that was usually whiskey) mixed with sugar, water, and bitters. The additional ingredients diluted and improved the flavor of the spirits, which were generally a bit rougher back then than they are now. (Even today, an expertly mixed cocktail will elevate a mid-level whiskey.)
Eventually, as bartenders began mixing drinks with gin instead of whiskey and liqueurs and vermouth instead of sugar, some nostalgic—or perhaps inflexible—drinkers would specify that they wanted their cocktail “Old Fashioned.” (Liquor snobs exist in every century.) Hence the name of the classic whiskey cocktail that, along with the Sazerac and Manhattan, laid the groundwork for whiskey’s representation in early cocktail culture.
Whiskey predominates in all three of those foundational cocktails, but it’s flavored (and sweetened) with small amounts of additional ingredients. So why should it be sacrilegious to simply drink a flavored whiskey on the rocks? There are varieties out there (infused with honey, vanilla, apple, ginger and more), that are well-balanced, smooth and delicious. Kind of like a well made cocktail.