First, the resin, aluminum or steel upright and top plates are NOT structural components, you can remove them all on a full pool (round) and NOTHING will happen to the pool, they provide stability to the wall - if someone pushes on them to get themselves out, the weight is supposed to transfer to teh associated uprights and to the ground from there.
My pool is 2 years old, my resin uprights and top plates are as nice as the day they were bought, there is NO sign of any UV damage such as drying flaking etc. If they all rot out down the road, replacing them will cost me 0 dollars because they take about 2 minutes to remove (for 1 top rail and 1 upright) and I can replace them without needing to do more than releasing one bolt per set.
Oh, BTW as an FYI, - My neighbour's pool is about 20 years old - Aluminum structure, steel wall, at each joint between top plates, he has resin or plastic caps, they are all in better shape than any of the aluminum structure on the pool, matter of fact, they look new and the paint on teh aluminum is so oxydized, touching it makes it chalk off, so somehow, I have a hard time believing the resin/plastic stuff will fail early.
UV protectants are added to just about anything that is required to be outdoors 24/7 nowadays, my Brother in law has a resin shed approximately 5 years old - still looks as good as the day he bought it.
The wall on the pool is not made from resin, all pool walls are either aluminum or steel, mine's steel with umpteen coatings on it.
I would NOT recomment Aluminum posts or top plates because they are not very strong - meaning, you have a 200 lb person pushing on them to jump out of the pool (instead of using the steps) and you'll have permanent damage (dents etc) to the top plate, you bang an upright with your lawnmower and the same happens, at least steel is a bit stiffer.
With the resin, it barely flexes under a 250lb load.