I haven't read most of the thread, so if what I'm about to say has already been said, then my apologies, but I'll address the premise of this thread with a few statements. I want to preface this with a stance explanation: This is not meant to be an inflammatory response in any way. This is not meant to be insulting or dismissive.

1: Failure is OK. Failing and learning is the very heart and soul of MMO gameplay. The stakes are so low in FFXIV that failing is not punishing in the slightest. Learn from each failure and do better next time. This is also the foundation of prog raiding, but obviously in a much more punishing circumstance.

2: No one should ever strive to remain at the same skill level from start to finish. While everyone has different skill levels in the game, and that's ok, when you start an expansion, you SHOULD be worse than when you finish it. That means that your skill when you begin the game at ARR should be night and day different from your skill when you enter the arcadion. Now, the starting line and finish line for each individual player can and will be different, and that's totally fine, but if you end with the same skill level as what you start with, then what's the point of playing the game?

3: Yes. The Arcadion, and the battle content in general for DT, is higher than the same respective content in previous expansions, but not to the exaggerated amount that the OP insinuates. It's arguable that the easiest Extreme trial would be Zodiark, and the hardest Arcadion fight is not even remotely close to the difficulty of Zodiark EX. I would argue that Warrior of Light NORMAL is harder than Zodiark EX, and that was a shadowbringers trial.

4: If you're struggling with the mechanics in a fight, there is absolutely NOTHING WRONG with putting a quick "Hey, I need to focus on mechanics next pull to figure them out" and then either JUST 1-2-3 or even don't worry about damage at all and just focus on learning the mechanics. In a normal raid, you don't have a rage timer. You can literally fight the boss for an entire lockout by just surviving and never doing any damage. If you need to focus on learning mechs and not hitting your damage buttons, then just tell the party. As long as you're actively trying to learn, no one is really gonna care. The few unreasonable people that would will get over it.

Simply put, properly designed games scale the difficulty as the game progresses. This is normal and good game design. The difficulty scale in DT FEELS more exaggerated than it actually is due to the decrease in difficulty that EW had. Rather than get upset that the difficulty is increased, your best bet is to stop worrying about failure and focus on learning and overcoming the hardship. Rather than get frustrated over the difficulty, I would recommend learning to enjoy the growth process.