Football pre 06-08 for england probably was of a higher level, but when you consider the players playing at the time for the english clubs and that there is a current rebuilding phase that's what you'll get weaker players. If strikers are scoring more nowadays because of the weaker defenders it's a case of you're only as good as the last opponent you face.
I'd also like to add that Barcelona are partially the reason why defences have weakened. While I wouldn't go as far to say people try to copy barca, most english teams have gone from pretty defensive humdrum styles of play when people in the prem first started using 3 man midfields(even United used it in 2008 and Chelsea started the 3 man midfield popularity in england) to more attacking styles of play post 2010 which has also coincided with barca's best period in their history(no coincidence here) and due to that you now see teams more attacking based and looking at possession football more. Be it Chelsea, even United.
The reason the standard of defending has slipped is partially because teams are more willing to attack the opposition too. There's also the case of Madrid and Barca basically taking key players off the english clubs around 2009-2011 period which has made the league at the top worse, then there's the other case of Chelsea who didn't replace the old people with youth so had to transition from the old guard to the youngsters in the space of 1/2 seasons. So I believe that all the clubs have had major surgery the past few years or are in the process of having major surgery.