I dont know this game well enough to really say anything.
If it is the way you say, it sounds bad way to handle the difficulty and i wholely agree.
Does the game have any progress beond the achiviements? Is the game run based?
If the achiement system is done in the same style that in The Last Spell, where the gameplay loop is build so that at the start the game is supposed to be impossible to win, but where after every run you get stronger in small increments until you are just barely able to beat a level and move on to the next one, it is fine. It means that by the design you are meant to loose and the progression of the game is that you get every time little further. Changing the game difficulty to easy and having the ability to get the same upgrades could ruin the gameplay loop.
But again. I havent played the game so i dont know. All i know that some games use roadblock enemies or level gates to make sure players have progresses enough to be ready for the next levels. Its not just to make game longer. It is also about to rythming the game. Devs can calculate that average player needs X tries to level up enough to progress in to the next part of the game and it can help them make sure players spend enough time in certain level, biome or what ever but not long enough to get bored to it.