A longer game can justify its length with new mechanics, exploration, encounters, and story, among other things
The question is whether the time it asks from the player is actually worthwhile.
"old games were made more difficult to compensate for their short length"
And? What's wrong with that? Video games would be much better today if they were shorter and more difficult.
A skill-barrier is a much better way of giving your game value than padding it out with filler.
The Batman outright has Jim Gordon boast that Falcone couldn’t buy all of the GPD, a statement rooted in the belief that the system isn’t corrupted or broken beyond repair, there are just bad apples.