It started being devalued when sky appeared on the scene, live football saturation and the PL money, then the expansion of champions league and the riches to be earned from that. This has led to the situation we have now where finishing fourth is a higher priority than winning the FA cup, or the likes of Burnley play a weakened side in the cup as staying in the PL is a higher priority. Trafford utd missing it completely one year too!
The decision to play the semis at Wembley really killed it for me, the whole magic of FA cup final day used to be about Wembley now it's nothing special at all. Also, moving the KO time and playing it before the season has finished - just not the event it was.