Shrinking single player is nothing new. I first heard complaints about short games ten years ago. There was a significant drop in game length between the 16- and 32-bit eras, and it just kept getting worse. I read an article around 2000 that noted that the average PSX game had gone from 12 hours to 10 in the span of a few years. Multiplayer and DLC definitely have a hand in this, but the main culprit is and always will be improved graphics. Developers want to spend lots of time making their games look nice, which explains both the reduction in length and increasingly common delays.
I do pine for the days of 15-20 hour single player campaigns, but you may want to be careful what you wish for. Developers do listen to their audiences, and their response to the complaints of shrinking games has been to pad the hell out of their games. A lot of things that gamers hate - backtracking, reusing levels, fetch quests and unnecessary minigames, to name a few - are dirty little tricks that the designers use to make their games look longer than they really are.
There's the catch. A 6-hour game is certainly very short, but sometimes that's just how long it was meant to be. Trying to force it to 12-15 hours will only dilute it. Think of a game like a speech - a ten-minute speech is certainly very short, but padding it out to twenty minutes to make it a more acceptable length will ruin it more often than not.