This has the potential for a compelling discussion regarding the talk Sabenaboy linked.
A study on the impact of glass cockpits in 1985 showed issues with automation. A study on performance of pilots in 1995 showed first signs of erosion of skills. So the problem is not new and probably getting worse, although we do not have objective scientific data to support it.
If it is the NASA study, I have read it and attended a conference on glass in the cockpit decades ago. A couple of words and phrases used in the NASA report to describe the issues that stuck out in my mind are "loss of scan", "complacency", and the one that perfectly describes where we are today "automation atrophy." This was three decades ago, before the A320, with all of its 'new' automation and control 'laws', was part of the worldwide fleet, yet the concerns, if we look at recent accidents and incidents, are identical, are they not?
All of this outstanding automation we have today, taking away much of the workload, was promised in the beginning to foster more situational awareness, but by taking the pilot out of the 'loop', I feel we've actually gone a bit backwards on that front because of the automation. I think it is due to the implementation and use. To that point, I found this to be a key quote from the 1985 Ames Research Lab report:
Early in the project we recognized the need for guidelines and principles for the design, operation, and training for cockpit automation.
This is could have been a key turning point, yet here we are, 3 decades later nearly in the same conundrum that faced new crews on aircraft like the 757, which was the focus of the study. I would imagine these ideas were difficult to implement since we had entrenched design philosophies at both the major manufacturers - both unwilling (or not having a clue how) to change their 'system'.
The study I referred to earlier came to the conclusion that there is no evidence that the deterioration of skills was solely because of a lack of practice. Other contributing factors could be in play, and this applies especially today.
The only part of M. Drappier's talk I disagree with a bit. Manual flying requires "muscle memory" and hand-eye coordinated skills - lack of 'practice' as the author states is certainly not the sole factor, but it's got to be nearly all of it, as it relates to the current state of manual flying skills.