At least on the Scamp
the lower floor add little or no support to the shell.
The lower floor is ... just a floor.
The floor in the front and rear is important, however. The bonding of the floor to the shell is all that holds the shell in place. The sides (from the frame rails to the shell) add some very small support as they are not really solid enough for the addition of support. (and are only attached to the frame rails with a few sheetmetal screws.
The front and rear supply almost all of the strength as they sit on top of the frame and are the primary support.
Personally the problem with OSB is that is has a lot of glue and water easily penetrates the substrate and rots.
Plywood will also rot and the real problem is that Scamp
waterproofs the bottom and not the top and water will find a way in and it WILL rot the wood.
In my opinion if you are going to the trouble of replacing the floor you should fiberglass the top of the floor and attach that floor with "tabbing" solidly all the way around the shell.
In my opinion there should also be some additional support on the sides to add strength to the shell all along the sides to prevent the dreaded door sag and distortion of the shell due to no meaningful support all along the sides. The door side is even more weakened by the door gap.
Replace the floor, Fiberglass the floor to keep the water out of the wood. (By the way Polyester resin is not very good at maintaining a seal on the wood and if you look at the millions of the fiberglass boats with rotten wood stringers and transoms you will see what I mean.)
ALL trailers will eventually leak from somewhere (even new one) and if that water sits on the floor it will rot if it can get to the wood.