I asked myself the same question but digging deeper it becomes clearer. Take it with a grain of salt because I haven’t actually used Economy but this is what I’ve noticed so far in terms of differences or benefits/limitations.
Generally, yes I believe you could make all of what Economy offers with only Cloud Save. Well, that is if you add Remote Config/Game Overrides and Cloud Code into the mix.
Think of Economy as the Inventory of purchasable items for each player. Whereas in Cloud Save you store only the items that are “common” and already exist in the game or were made available for free with content updates. It’s really important to keep the distinction between what’s free content and what is “paid” content, even if it was a “gift” for free. Typically you want to present these sorts of items differently to players as it entices them to actually buy (more) items.
Economy’s Inventory allows you to store “instance data” so you can record how many uses are left, how degraded the weapon is, or what kind of ingame upgrades were applied to the item (think: gem sockets, companion/pet level and learned skills, visual alterations).
Economy allows you to add more items and currencies but make them available ONLY after you publish them. Assuming a Cloud Save only solution you’d have to combine it with Remote Config to “publish” at a given date.
If you want to do real-money purchases the items have to be defined in Economy anyway, otherwise you’d have to run Cloud Code methods that perform the purchase operation.
You will also have to write custom code to record the date and purchase history of items if you try to implement trading only with Cloud-Save.
You can look up a player in the dashboard and see what Economy purchases were made and what the player’s currencies are. If you were to store those only in Cloud Save it won’t be separated from any other cloud save player values. If you look up the player’s cloud save data in the dashboard you may see some player data keys first, then comes the “Coins” currency, then some more data keys, then the “Gems” currency, then more data keys, then the “Inventory” with both free and purchased items.
If you have lots of items with lots of custom data associated with them, OR if you have very affluent players buying everything they can, you may run into the 2000 keys / 5 MB limits with Cloud Save and may be forced to split your purchasable items “database” over several Cloud Save “custom items” (each with 2,000 keys / 5 MB). But if a player can ever use up the entire 5 MB space for his own Cloud Save data then … well, end of the line when it comes to buying more. More likely than not you have not considered that case and the invested player only receives some frustrating errors like “limit reached” or “not enough storage”.