It's utter lunacy to blame one man for the club's failure.
Kenny left the club 22 years ago, is he at fault for all those years in between him returning 2 years ago? Surely some of the blame has to go on Souness, Evans, Houllier, Rafa, Hodgson and Rodgers. What about the owners who failed to build a new stadium, got lost in promotion and advertising, thinking we could carry on as we had been doing, not willing to fund the players that the managers wanted etc.
As to Kenny not adapting to the new league system, how do you explain him getting Blackburn promoted back to the top division for the first time in 26 years, finishing in the top 4 the following season and winning the league in his 3rd season. He also finised 2nd with Newcastle in his first season, qualifying them for the Champions League and made it to the FA cup final before being sacked.
I'll tell you what has cost the club over the last 2 decades - failure in the transfer market.