I can imagine building a new GRP boat with a wooden deck. GRP makes an excellent hull material, as it takes compound curves rather graciously. The compound curves, to a large extent, make up for the material's low stiffness to weight ratio. Also, hulls typically have very few items bolted on later after they are completed.
Decks, on the other hand, have little or no compound curves, and have items bolted on long after the vessel is completed. Also, a GRP deck usually needs to be sandwich-construction in order to keep the weight within reason. Adding and subtracting hardware to a sandwich deck requires a number of precautions which are often overlooked. Also, there is the problem of the sandwich construction failing, due to the before-mentioned oversights and or the deck taking a load the designer did not anticipate. Once the sandwich structure fails, it is very expensive to repair. The GRP on either the top or bottom must be removed, as must be the core material. New core material must then be added along with a new GRP surface. And one must hope all this bonds well.
A plywood deck has none of these vices, however it can rot. But the rot is easier to see in a deck than it would be in a bilge. Once spotted, it can be locally removed and replaced. I also believe it is easier to reinforce locally for heavily loaded hardware such as winches and cleats.
If I had an old GRP boat with a failed sandwich GRP deck, I would try to repair it. Such may be less expensive than removing it and replacing it with a plywood one.
