I took the exam last year, but my thoughts are that old exams and Decks are absolutely enough to prepare for the exam.
Before passing judgement on whether or not the exam was impossibly difficult, I'd wait to see what scores people get. Scores for the written won't be out for 4 more weeks, but I'm guessing they won't be any different than in years past. Some people will fail, most will score between 80 and 90, and some students will score above 90.
Can a student simply pull out the Decks 2 weeks before the exam and score a 95? Doubtful, but I believe they can do that and score an 85 quite easily. Can a student who struggled through all their didactic work pull out the Decks and in the same timeframe score an 85? Possibly, but I'm fairly sure they could pass by using the Decks for two weeks.
So when you talk about "preparation for the exam," we might be comparing apples and oranges. Part I is truly an easy exam, and it's probably that way on purpose. If you have a light on upstairs you'll easily pass, but you'll have to put in some work (in your didactic courses AND in your test study) in order to do really well on the exam.
That's my thoughts.