Confused about demi vs espagnole vs glace

From what I can tell, the recipe for espagnole is the exact same as for demi, with only the roux being the difference. The recipes i have seen are beef, mire, tomato paste, and even the same sachet as for demi. Is the only difference between the two really just the roux and the reduction ? If the roux is indeed the only difference, then why would you combine stock and espagnole to make demi ? Couldn't you just make demi, and add the reduced amount of roux to it ? And if glace is just demi further reduced, why doesn't everyone then make glace and use that as a base instead of demi? If it's more concentrated wouldn't that end up being the most space and consumption friendly ? Ie now you can have "twice the demi" in the same amount of storage space ? submitted by /u/standardtissue