Macbook Pro ...... such as thinner frame (a` la Macbook Air)
As the name implies "Pro" is intended for a more demanding customer base. Therefore I suspect you will find the reason the MBP chassis is the size it is due to technical or mechanical constraints (e.g. CPU, heat generation etc.)
Somehow given their trend, I don't think Apple would deliberatley miss out on an opportunity to go "smaller & lighter" if they could.
with SSD and no more HDDs with mechanical moving parts.
You obviously haven't visited the online Apple store in many years. The option of a SSD HDD custom option has been there for a long time now, well before the current refresh.
The reason it is not there as a standard option is for the very good reason of keeping the price point down.
The issue of SSD being more reliable than mechanical is debatable by the way.