To this old timer, the path downhill started with 2nd edition.
My philosophy has always been play what you enjoy. Now if I could talk my current group (my wife and her 16 year old grandson) into playing Chaosium's Call of Cthulhu (5th edition rules) or maybe a 2nd edition Gamma World game set in the world of Thundarr the Barbarian - I would be very happy.*
As for 4th edition... I'm not their target market so I'm giving it a complete pass (same as I did with 3.5 D&D). More power to those who do go that route. I will stick with the games I know & love.
* For the record, we have played Villains & Vigilantes (2nd edition), 3.0 D&D, and 1st Edition AD&D.