Actually, McDonalds does make many of their own sauces. Particularly their own mustard and bbq sauces, which, I actually got to participate as an observer of someone running a taste test of McDonalds BBQ sauce with several other sauces and McDonalds did quite well.
On a different but on topic tack, there is nothing inherently wrong with taking a commercial sauce and modifying it. There is a risk that the formula might change, which is certainly a risk that can occur. Although, how many of us use Worcestershire sauce, or make our own ketchup that we base sauces upon. In many other types of restaurants, there are basic sauces that are often used as a basis. A good example is Lee Kum Kee, which at one time, before it became a staple of the Asian food aisle in supermarkets everywhere, owned nearly 90% of the bulk sauce business in Chinese restaurants in the U.S. It is not so unusual to use premade sauces as we all might suppose.
All this being said, I would make my own sauces, as I prefer my flavor profile and the ingredients I use.
Whip It Off, Chambers!
"perhaps...but then again...maybe not..."