I think you are putting too much emphasis on Leicester and Palace - and using their performances to justify your argument that all teams are getting harder to beat. Leicester are a phenomenon - and in many ways a one-off this season. Pardiola always has blue streaks - but Palace are not pulling up any trees this season now. Lower teams have undoubtedly become more organised, as their managers see that the way to unsettle teams that are technically better is to defend with better discipline and to press hard, but that's what football does - it moves in cycles - and its for rich teams with supposedly world class managers like Arsenal to adapt their playing style to cope. For every Leicester or Watford or West Ham this season; there is an Everton; Chelsea or Manure shedding points that they would not have been expected to in past seasons - plus we have taken points from Manure and Citeh that we have failed to do recently, so I'm not sure that our points total so far can be simply laid at the door of a generally improving lower half of the table.
I suppose the more interesting question is why Wenger cannot cope with the 'Sunday football' that you describe. To a degree this is nothing new. The EPL has always favoured footballing sides less than 'blood and thunder' - and its not as though our manager is not used to it FFS! And its even more frustrating when earlier in the season we seemed to have realised that a high pressing, passing game can be flawed and expose us to the kind of counter attacking football that has been our undoing many times this season, yet lately have abandoned this. I accept that to say we should be easily winning this league is to expect that the manager marshals his resources better to face the football that we are seeing - but I don't think this expectation is any less valid for it.