"Are you arguing that maintenance and upkeep are not important to a GRP hull and deck?"
Pretty much.Mostly cosmetics is affected.
A solid GRP hull left open to the elements does not suffer harm from rain or snow over the years.
Physically GRP does will last a long long time with out structural damage.
There are Boston Whalers from the 60's that have been outside, uncovered for 50+ years.The wood may be dust , but the hull is still sound.
A larger boat will have huge , mostly cosmetic problems , inside the interior may be shot , and the machinery and systems may be ruined , but the hull will still be sound .
The hulls of wooden , steel or glass over ply will probably be long gone with no maint abuse, but the solid GRP does survive , as do the more expensive cored hulls .
This makes the selection of a fixer upper far easier , paint is lots simpler than re laminating a deck.