I agree that useful things found on islands are scavenged. If the raft was there and close to the beach it may have been washed away by the time anyone got there to scavenge it.
Items like parachutes and inflatable life rafts may blow away in the wind very quickly(if they were even there and taken out and used)
Also regarding the land search,
Clothing disintegrates over time
Small metal objects like weathered watch parts may be so small, they may be very difficult to find.
As discussed other durable and useful items get scavenged
So what is left on the island that could have survived and not be scavenged, and can still be found?( which may be the point of diminishing return argument)
Broken glass from a compact, broken lotion bottles and broken freckle ointment bottles make sense to me. Non corrosive, small so not seen easily, and not likely rolling away by wind force and if broken not so useful to a colonist. What TIGHAR found fits the candidates of items that could have survived weathering and were also not likely to be scavenged. But to find those artifacts, TIGHAR did a lot of detective work to select the Seven Site for a more detailed search.
The TIGHAR research based on interviews and the Bevington object, and land-able area already suggests
the camp zero area to search in more detail as already discussed.
The Debris field trail followed up the sea slope may suggest an even more defined area to do a land search just as the aircraft debris washed ashore suggested where to look in the sea based on where it was found and the currents. The Land and Sea searches help each other. Myself, I’m still interested in a land search even if they find the plane in the debris field. It’s the story of what happened and the work TIGHAR is doing that I’m interested in. I understand the arguments on diminishing returns. There are ways to narrow the search for the needle in a haystack that increase the chances of a return on the invested time. I think TIGHAR’s approach is doing that very well. Especially with the limited time they have had to spend actually searching on the island