The only reason to go smaller is your current frame doesn't fit. Longer stems can also change the handling.
The saying 'the Pro's do it" is cause years ago (and some now) frames were custom built for them, the frame could be
smaller (seat tube measurement) and still have a longer top tube. Pro's also ride much lower than 'other' cyclists,
which can make a frame look smaller.
If your current fram fits, you are comfortable on it & it handles like you want, there is NO gain going to a smaller frame.