Interestingly enough Escoffier discusses that "many chefs of the old school do not permit the use of glaces in culinary preparations". I wonder how "old" the "old school" is that he refers to, as it would appear that chefs today refer to Escoffier's methods as "old school"??
This is a great observation. I suspect Old School to Escoffier was the 17th and 18th century, when the chefs were in service to the kings and aristocracy, and didn't have to deal with the vulgarities of a la carte cooking, restaurants, the middle class, and budgets. Back then, naturally thickened jus (called coulis) were prepared as sauces for roasts and sautees. This was done a number of ways, but it all amounted to cooking a lot of pieces of meat in order to get enough sauce for one piece.
In a sense this is what we do with stocks, but we use bones and much cheaper cuts, and long cooking and reduction in order to concentrate flavors. This is just not capable of achieving the fresh complexity of a natural jus or coulis.
The simplest approach was to roast multiple pieces of meat on spits, and eventually collect enough jus to serve with the final roast.
Sometimes they would squeeze all the juice out of these pieces of meat in a big iron press.
Another approach was to make stock from the poaching or braising liquid from a piece of meat, and then to poach another piece of meat in that liquid. This was called a double coulis. A tripple coulis might also be prepared. I don't know what all these extra pieces of meat were used for. Given to the dogs, maybe, or the kitchen staff.
At any rate, after the revolution disenfranchised the chefs from their patrons (and their patrons from their heads) the middle class was born, along with restaurants and a la carte cooking. The old ways wouldn't work with the new time and budget constraints, so chefs created time and money-saving shortcuts to simulate the favor of the grand old coulis.
Careme and the chefs of his generation created these shortcuts (including the family tree of classic sauces and glaces). It's worth remembering that the extravagant preparations of haute cuisine (like double stocks made with staggering amounts of meat) were actually born as shortcuts.
Escoffier's methods from a generation later no doubt simplified and economized Careme's shortcuts, just as the mid-20th century chefs simplified and economized Escoffier's, and the chefs of Nouvell cuisine simplified and economized theirs, and as our generation is currently simplifying and economizing theirs.
This isn't to say that a restaurant that makes a demiglace by reducing stock made from bones and vegetable ends is doing anything wrong, but it's folly if they don't recognize their methods as a shortcut on a shortcut on a shortcut, many times over.