Having been involved in many, many surveys and having been on and in the bilges of lots of boats, both power and sail, I can assure you that what I've seen is good cause to not use wood. Its why I don't.
On almost every single boat we've come across at least one or two backing plates that are rotted and or punky, and on the way out. Perhaps 55-60%+ of the wood backing plates I've stuck a meter or an Awl to have been bad.
Fiberglass or composite backing plates have been around what for the last 5 minutes ?
Actually there are many yards in Maine that have been doing and offering this for years! It is not new by any means. Up here a fiberglass backing plate is nothing in expense when you have a 20-30k varnish bill each year..? Also most of the top Maine builders have been using solid glass where seacocks and thru-hulls penetrate for a loooooong time.
Just one more way to spend extra money when you don't need to.
Ummm have you priced marine ply lately?? I can make a 2X2 solid sheet of fiberglass in about 20-30 minutes and for a comparable cost to marine ply.
Wood backing plates have been used on fiberglass boats since the 1960's. I have never heard of a failure if looked after under the normal course of maintenance.
And I have yet to meet even a DIY owner who takes a meter or an Awl to his backing plates to actually check them. I have seen many that still appear fine to the naked eye but that are very wet and if you poke the Awl in the right location you'll find wood softer than Balsa..
Even the OP states his wood backing plates are "ok" after 20 years of use. Sometimes we don't need to fix something with something new if the old one or way isn't broken.
This is not surprising as Cape Dory normally used Spartan bronze seacocks that are through bolted thus giving more integrity and less flex induced into seacock the system. I have seen direct corelation of dry wood backing plates to the level of install meaning I've see more wet backing plates on boats with only a thru-hull with a valve screwed to it than I have with boats that have flanged seacocks and through bolts or lags into the backing plate.
Here is a photo of one of my home made, hand laminated backing block sheets. Easy, permanent and it will last as long as your hull.
As always feel free to read these for more information and photos:
Seacock & Thru-Hull Primer/Pre Information (LINK)
Replacing Thru-Hulls and Seacocks (LINK)