I think the fervor over this game's length is stupid, in that its lead to an endless stream of silly, poorly reasoned arguments over what the "proper" amount of length/content is for a single player experience.
As many people have (accurately) pointed out, there are countless classic games that could be completed in an afternoon. The vast bulk of 8/16-bit games were incredibly short. I remember my original playthroughs of A Link to the Past (in 1992) and Super Metroid (in 1994) only taking about 4 hrs a piece.
But in referencing those games, you get to the heart of what DOES irk me about The Order, what to me is a far greater failing on its part. We tend not to remember those myriad classics as being so brief because they were so damn fun to PLAY, and in everything I've read about The Order, positive and negative, the main takeaway is that the real point of the game is the "experience". The actual play is almost just there to be there, to justify its existence as "Video Game". It's like the gutter in a comic strip, an inconsequential blank space intended to convey the passage of time between the moments of import.
I hate it when games are designed with that attitude. I hate it when games treat the player, who should at very least be an equal partner in the proceedings, as an inconvenience, an ornery child to be corralled, disciplined and silenced when the adults are speaking. I fully appreciate diversity, and that different people enjoy different things and experiences, but I very deeply believe that if you create a video game, it should be judged first and foremost on its merits and imagination as an interactive experience. I mean, I love the Metal Gear Solid series, which are also dense with narrative, but the moments between the talking are elegantly designed and highly replayable, true games with malleable, reactive worlds and a respect for the player's agency.