Man, people have been complaining about game length since the PS2 launched. Back in 2001 it was Onimusha that caused all the turmoil. It was considered one of the first few big PS2 titles and it took "only" 6-8 hours to finish. This was before the price bump too , so you went out and paid 49.99 USD (lucky yanks, it was 80$ canadian for a ps2 game in 2001) and you could marathon the game through in a day if you so desired. There were a few things extra to do but , coming off the PS1/N64 generation of 15-20 hour titles being the norm it was a big complaint.
Last gen we got the privilege of paying 59.99 USD for new games that looked 2-3 times better than the previous generation and game length seemed to average out at around 8-10 hours for any single player titles (counting 1 run through on normal difficulty that see's the player watching all of the cutscenes and possibly dying a few times). Now, we've got games that have again bumped up the visual quality 2-3 times and so far , game length hasn't suffered - The Order is merely the end game of what happens to a narrative driven title with the presentation set through the roof.
Could it have been longer ? well sure, but it would have been at the expense of the visuals or lead to more monotonous level design. Why ? well, they aren't going to create tons of expensive new art assets just to add new and interesting levels to the experience , instead padding a game like The Order out would have involved building larger levels out of existing art assets and re-using the same few enemies and encounters even more than I'm assuming it already does.
We are also forgetting another piece of the puzzle here. This expectation of game length is a strange thing in gaming , it wasn't always there. From the 70's through to the mid 90's games tended towards shorter , challenging experiences. The marios, Ninja Gaidens and megamans of the world back in the were all titles that could be completed in 2-3 hours on a first play through. Games which were even shorter tended to have other walls to prevent an easy win - limited lives and continues, 1 hit deaths. As well, some titles that could be finished fairly easily would also have difficulty settings , play through them again and again.
Some of you might be thinking - "yeah well, those old games only cost a few bucks on virtual console". Well, they didn't cost that when they first came out. In fact paying 50$ for a game that last 2 hours on the NES was considered a pretty good purchase. Every game cost 50 $ until pretty late in the NES life cycle. SNES / Genesis games were still mostly the same length for the same price but - they looked and sounded better and with the extra buttons offered on the controllers - played better too.
Basically, the short length of the Order shouldn't be a factor in whether it's cinematic experience is worth it to you. Want a longer game ? go play the boring and repetitive Destiny instead and waste 100's of hours replaying the same short stages to get an ugly helmet.
There has to be room in the gaming world for titles which don't fit the bill of "6 -1.5 hour long stages with 1 hour of cutscenes + the same multiplayer you've had since doom". Not everyone has to like playing an interactive movie or even give it a chance but stop complaining about it costing to much. It doesn't deserve that.
A final aside - at 6 hours of length, the Order is almost like a trilogy of films or even a multi part HBO mini series. A season of game of thrones on bluray costs 60-80$ when it comes out and 3 bluray movies are also worth 60-80$. So what I'm getting at is , even when I look at the price - compared to other mediums it's still pretty much par for the course.