Well spike teams in general are more skill testing than HP/RCV boosting teams, which plays well to the Puzzle part of P&D.
In general though it's a consequence of the inflationary power creep. If you've noticed, every time GungHo accidentally makes an OP monster (Horus, Lucifer), they raise the baseline difficulty to keep people spending stones on their dungeons. They can't balance the monsters by nerfing them either, because the whales who threw thousands of dollars to +297/maxSL/(*) the monster are going to get pissed.
P&D has started to run out of steam in Japan; their new user acquisition rate has slowed (the X million user celebration events are getting spaced farther and farther apart) and the amount of money they make month over month has reversed direction. I think outside of Japan they are growing but still fail to break the Top 10 unless a Godfest is going on.
I think as players tire of PAD they'll try out other F2P games, but the issue with most of the ones I've played is that they are either shit from a fundamental gameplay perspective or they aggresively push IAP onto the player. So instead of playing another F2P game the userbase will just exit the market altogether.