There is a pretty wide gulf between "every class or race or spell (or anything else) has to exist in every setting" and "setting never change from its first printing".
Sure, you don't have to add everything, but the default answer shouldn't be, "Well, it wasn't there thirty years ago, so not now" either. If adding to the something makes sense and is in keeping with the setting, then, IMO, it should be added.
Alchemists and mad scientists in Greyhawk? Yuppers. Fits right in. This setting always played fast and loose with genre lines. Like I said above, you have at least two crashed spaceships in the setting, a giant mechanical spider, and a wizard most known for carrying six-guns.
I'd say that its a pretty small jump to add in alchemists to the setting.