This is false. People get better by gradual increments in difficulty, not massive leaps. You may prefer that method personally but I can guarantee you never learned something hard by just throwing yourself at the most difficult part and overcoming it. Anything hard you learn, you do so by breaking it down into smaller parts, you learn those pieces and gradually build yourself up to the point where you can do something hard.
Where this game needs to improve is in providing that scalable difficulty and not provide sudden jumps in difficulty and pray the players will stick with it. Start with Sastasha, make the aoes there a little (25%) smaller and make the cast time a little (10%) longer and keep it that way through copperbell, basically enough time to get used to group dynamics and how your class plays. Thereafter, in each dungeon, increase the aoe size and decrease the cast time until castrum/praetorium where the aoes and cast times will be as they are now. In lv 50/60/70 dungeons decrease the cast time by another 1-2% and in current EX decrease it another 1-2%. This allows people to get used to harder content without overwhelming them with sudden spikes in difficulty. The best part is the ability to make bigger/smaller aoes or longer/shorter cast times is that the means already exist in the game engine; it doesn't require any special programming, just a retooling of existing dungeons slightly.
Sudden death mechanics are also bad for learning, all sudden death teaches you is that failing it kills you. It provides no opportunity to repeat and learn beyond a raise or reset. Instead of sudden death it should halt progress; you can't complete this fight until you do X correctly. This is effectively what a DPS check is, you can't proceed with the fight until you figure out how to do enough DPS and it lets you improve as you do it. Ultimately if the improvement takes too long, you'll fail and have to try again. Learn how to do your rotation right and you get to continue. Going hand in hand with this is the need for better feedback to the player, they need to know how and why they failed so they can do better next time. Sudden death tells you neither, just that you failed.
It wouldn't hurt to see more prevelant use of Trial/Savage mechanics in regular dungeons either. Players should have the opportunity to be introduced to those outside of savage where failing can wipe the group.