Why do we play Maplestory?

Sure, cards lose their value but the fact that they actually have a tangible value and are some sort of investment - good or bad - has really made me look at MapleStory in a different light.

I picked just this point out of your entire post because I am really sick and tired of people bringing this "at least I have a tangible item" thing up.

MapleStory is entertainment. Entertainment, by and large, does not leave you with anything tangible. Whether you pay $5 for a movie ticket, or $5000 for a cruise around the world, when the fun ends you have nothing but your memories. You can enjoy watching a ball game for an hour, or using a season pass to Disneyland for a whole year, but either way, when the fun ends, you have nothing but your memories. There's nothing fundamentally wrong with that.
Yes, there are hobbies and pastimes that leave you with something tangible for your time and money, but most entertainment is not like that. You pay to enjoy yourself for a time, and enjoy the memories thereafter.

So, just because the investment in Maple leaves you nothing but memories - and friends if you're lucky - is no reason to stop playing it.
If the enjoyment you derive from the game is not worth the money or time it requires, that's when you stop. Just as you decide whether to go to a concert by weighing how much you expect to enjoy it vs. how much the tickets cost.