Just because you change a character past their original state does not mean they should be the yard stick.
Using the context of your post, I think you may have meant, just because you change a character, doesn’t mean they can’t still act as a yard stick. Please correct me if that’s not what you meant.
Um… yes it does? If you change the length of a yardstick, it’s not a yardstick anymore? If you change the length of a yard, how can you use it to meaningfully measure anything? That’s what yardstick means… a fixed object you can compare to another objects and get a meaningful idea of their length.
Yardstick characters are incredibly important for game balancing. If you change Tracer – buff, nerf, or otherwise – it becomes nearly impossible to see if the game and characters have the same balance as it did when it launched. Powercreep is a very real thing, and keeping at least one character as a baseline allows for the creation of new characters that are in line power-wise with the existing ones.
If Tracer is too strong or weak, it’s not Tracer that needs fixing – it’s everyone around her. In my opinion, something like buffing the supports (namely, Ana) to make them more survivable against flankers like Tracer would make the game balance a lot healthier.