So how do I justify this? Well, I've come to the conclusion that Adventure and Frontier are basically the same thing with just a few exceptions: Frontier's story is slightly darker, the lack of Digimon partners enables the human characters to do more than just stand on the sidelines during the fights, and the script just felt a little more... put-together. So, to be frank, by season four, the creators had gotten their shit together.
Now, I love Adventure. I think it holds up over time, which isn't the case for all of my childhood loves. Frontier is the only season of Digimon I've seen that I don't associate with my childhood. I watched it to completion for the first time when I was 13 or so. So that frees it up from a lot of nostalgia bias I could be feeling. In a lot of instances, Frontier follows Adventure's story and characters to the point where I would believe you if you told me this was actually a remake.
Frontier took what I already loved about Adventure and improved upon it... in all ways but one. For a show that was successful enough to be granted a FOURTH season... you'd think they'd be willing to take a risk with the animation budget.
