A post on Steam from Frozenbyte addresses the amount of gameplay in Trine 3: The Artifacts of Power, their just-released platformer sequel (thanks DSOGaming). They explain that they did not reduce the game's length to increase the demand for DLC, but rather suffered a bit at the ambition of creating a fully 3D game which caused them to burn through their development budget much faster than expected. They say they do not actually have and DLC plans, in spite of what they call a "cliffhanger ending," and admit that the negative reaction to the game's length has put the future of the series in doubt, saying: "The future of the series is now in question, as the feedback, user reviews and poor media attention has caught us by surprise." They do offer assurances, however, that they will continue to support this release, expressing confidence that issues users have encountered will be ironed out shortly. Here's part of the post:

Back in late 2012, we set out to do Trine 3 in full 3D - bigger, badder, better. We took a big risk with the 3D gameplay implementation - it was to be a massive improvement over the previous games in several areas. We have always been ambitious and this time our ambition may have gotten the better of us.

Trine 3: The Artifacts of Power has ended up costing nearly triple that of Trine 2 – over 5.4 million USD. We have squeezed everything we could into the game, there's nothing left on the table. We initially had a much longer story written and more levels planned, but to create what we envisioned, it would have taken at least triple the money, probably up to 15 million USD, which we didn’t realize until too late, and which we didn’t have.

So we did not intentionally make the game “short” as many have said in order to make money off of future DLC or whatever. We tried to make something too ambitious, and it ended up financially impossible. What we sold on Early Access was the “realistic” vision and what we promised is what we have delivered, in our opinion. The finished Trine 3: The Artifacts of Power game might not be as long as we hoped initially, but something we are very proud of nonetheless, and generally around 6-7 hours is what we think new players will spend with the game on average. We're aware that some players have completed the game in less than that, 4-5 hours for example, and we accept that. I'm sure you can speedrun it much faster too, but so you could in previous Trine games and in most other games too.