I have had similar conversations in a lot of CCGs, so let me lay out some of the logic from previous discussions for you.
First, in a physical TCG you have a limited production run of cards. Regardless of how big the production run, those cards are by their nature finite. If there are 12000 of a card printed then there are only 12000 ever. Physical attrition further lowers that number. This allows for those cards to be collectible items with perceived value appropriate. Owning one of them is something semi-unique and interesting. Trading works well because of the limited numbers. Everyone can not have one of the card there are only 12000 of. Studies are actually conducted by the big TCG companies to decide based on player base just how big a production run should be to ensure obtainable desirability and value retention.
When the TCG concept is brought over to an electronic format like GoW that concept starts to break down. We can all open chests which contain a simple roll on a table. There are not finite numbers of anything in the game. From a simple hierarchy, monster cards come from Keys, keys can be bought with gems, gems can be given out for doing nothing more than simply logging into the game and having a proc chance fire off while collecting gold. This means that the number of gems is infinite, therefore the number of keys is infinite, and therefore the number of any given monster card is infinite. There are several other paths equally infinite to keys as well. Given time everyone is capable of possessing every card in the game.
When taken on an individual scale there will be cards you get before other cards, making the missing cards more desirable (to you). This provides the incentive to continue progressing in the game, to continue logging in. Your next shot at the appropriately sought after shiney is just over the next horizon. This also keeps you interested in each chest you open, because “this could be the one!” A major point here is that whatever you are short on, someone else is probably short on something else and considers that thing common. When trading is introduced these vagaries of luck are eliminated and when taken across the aggregate of the community leaves a lot of extra cards to be traded around. This means everyone completes their collections very quickly thus removing a large point of progression in the game.
This also introduces a secondary effect wherein the trading place becomes the premier source for collection of cards rather than “the wild”. People begin to feel pressured to use the trading place to finish their collection rather than playing the game. Loot rolls from chests become less important and thus less interesting. This leads to a general drudgery within the game that while it can be worked around is incredibly difficult for a game built on the general precepts that GoW seems to be built upon.
If the cards were limited time cards of which a theoretical finite number existed and if those cards suffered some level of attrition then trading could help without providing all these negatives, but in a game where card numbers are basically infinite and the selection of cards is relatively small it would lead very quickly to stagnation within the game.
It is an unfortunate thing we see played out in many of the various card-esque games on the market today. Some have trading, many do not. Those that do generally have limited numbers of cards (new cards released weekly with no way to pick up the missing ones later), or have a different mechanic that is keeping people playing, or possibly are already beginning their stagnation path. Solving the riddle of appealing to a huge base of semi-casual “come and go” type players while still providing that incentive for self-progression at the same time as you try to keep the completionist crowd happy isn’t easy and arguably hasn’t been accomplished yet.
I don’t take anything away from the devs for GoW. Perhaps they will be the ones to crack the code. But I do recognize the monumental task it would be to do so effectively. Frankly I am not sure I would consider it worth it since it would take re-designing the reward systems in the game at a pretty fundamental level.