"Less than 15 hours" could be a decent length, Deadfire's "slightly more than an hour" is not.

 

Don't compare apple and oranges. Deadfire in 1 hour and 15 minutes was a developer speedrun without exploit, not normal gameplay. BG2 speedrun record without exploit is also around 1 hour as well (with them it's 21 minutes).

 

If you do main quest only without ridiculous speedrunning stuff I'd be surprised if it took more than 10 hours.

Now that I think of it there is something interesting in the ME2 comparison.
Even though to me the ME2 story has always felt like just a random reason to gather a squad so you get to do all these cool companion missions and loyalty missions, the final mission is actually fantastic. I mean it's utterly shameless and childish sure, but it has everything to do with what you have been doing all game. From the start of the game you know your goal: take on the Collector homeworld. So you know why you are gathering a squad, upgrading your gear, upgrading your ship and taking care of all the emotional baggage of your squad, because you're going on a suicide mission. And everytime I've played ME2, it's actually felt great starting the suicide mission. Everything is ready, let's do this ****. And the outcome of the mission is actually affected by everything you've done earlier in the game. Writing wise it might be very uninspiried but execution wise it's brilliant. (Excluding the abomination at the very end.)

When it comes to Deadfire, I'm pretty sure I'll replay the game a couple more times at some point, and I'm excited to see all the DLC stuff, try Berath's Curses if they end up making those, build ridiculously OP characters and just generally screw around in the archipelago, but I'm not sure I'll ever finish the main quest again. There's just no reason really, it doesn't have anything to do with anything. But again, it might be just me, could be other people cared about the main questline a lot more than I did.