Is it possible for my humerus to still break down further after having a plate and 5 screws put in 10 yrs ago?

No: The bone or the hardware can break but only if you have another significant fall. The bone usually breaks below or above the hardware.

Yes: You can get a fracture at the edge of the plate, because this is a stress riser. This is the dilemma - there is a risk of getting a fracture below the plate, but removing the plate entails the risks of another surgery, and for a brief period of time the risk of getting a new fracture through one of the screw holes.