First of all, if you think a microscope is kinda important in medical diagnostic processes, then you are right. It is, but I think the definition you provided for the term microscopy needs to be modified a little. I would say, it is not to investigate small things but rather trying to see things which can't be perceived by the naked eyes. Of course, previously we can describe it as a technique to view all kind of microorganism but in 1999, scientists have discovered a bacterium which is quite large and can be seen without using a microscope.

Microscopy is, therefore, the science of investigating small objects and structures using a microscope.

I mean, if the object is small but still can be seen by our naked eyes, we wouldn't need a microscope, do we?