Did the game peak too soon? Was there not enough to drain out of different play styles? Genuine question, as I never got as familiar with it as I with Radiant - and that's one thing you could give Radiant, is that over time your play style changed as you figured out new chains. Perhaps not any more. Perhaps video kills some of that off - I don't think it was long after Ikaruga came out there was a 1p twin-stick video - and when a game is "solved", it takes some of the magic away from the experience and at that point it feels like you're drawing over a photograph.
Beautiful, beautiful game.
You could say something similar about DOJ, though. There's only one path if you want to chain an entire stage, right? So in that sense, DOJ can also be solved. I think that with either of those games, even if you do solve them, there is enough difficulty in terms of stamina and reflexes that it is a very difficult game.
Personally, I love Ikaruga, and I remember back in the day people here would rip on it, saying it wasn't a "real" shooter. Sure, it's not a bullet hell shmup, but I don't think it ever claimed to be that. It's a huge challenge, and I don't mean "It's difficult", I mean that Treasure were basically challenging gamers "I bet you can't beat this."