Developers take note: go backwards to go forward, give people more reasons to play Hard mode and above difficulty setting.
The best thing to do, depending on the game, is to communicate that to complete the game properly including going onto the true final level and boss, and to unlock a very special mode or even a new sub story level, or even to receive discount on a new DLC level, the player have to complete the game on Hard mode. That will force players to LEARN the game and be an awesome player. Again, it depends on the game, some games are balanced poorly by changing the entire game mechanics which is wrong. Let the skill of the player determine the reward. Coasties players that plays the game on easy mode, completes it, post everything they see on youtube or claim they are great at the game then to sell that same game the next day be damned. People didn't do that back in the 90s but you got to do something about it by providing more reasons/incentive for the player to get good at the game for many people are lazy and self rationales is no longer a common thing.
Lastly, and this is very important - communicate from the very title screen or before even the damn 1st game level starts what the player be rewarded if they play the game on a higher difficulty setting. Being cryptic or not communicating a all will do you no favour. For example (not the best example but still a worthy one) , Streets of Rage. If you play the game on Easy mode you cannot play the gane's final level. The gane will tell you well done you are great, but to fight Mr X, please now play the game on a higher difficulty setting to fight and receieve the true ending. Another example is Panzer Dragoon Saga, you must get a 95% completion to get the true ending. Sure, the game should have told people this feom the very start to give them a reason to invest the time in the game but the chance is there.
Rewards those that put the time and effort in your game, and give incentive to players to GET GOOD at said game. Gaming Excellences 101.