I think the trend will continue down for guitar playing, and for all musical instruments.
My theory is based on the fact that learning to play well is difficult and time consuming.
Popular music is by and large not the product of humans, moving muscles, energizing mass, thus generating sound waves. "Sounds" that we hear in pop music are in main generated digitally, usually with software instruments, in a computer.
Pop music is still uses the more traditional aspects of music composition, but only as a component of an ever expanding sonic palette.
Modern production is based increasingly on the ability to manipulate "sound" in a computer, and assemble it into listenable compositions.
The human voice remains one elements that is still generated by biological processes. But even the voice is subjected to increasing amounts of digital manipulation.
Learning to produce music in the modern style is also difficult, though in a different way from learning to play an instrument. Specifically, it is not a realtime process.
Given the finite amount of time and resources available to individuals, especially young people, it is inevitable that learning more modern pop production will be at the expense of investing in the extensive training needed to perform music in realtime.
This is compounded by the fact that the economic weight of the music industry is in the world of pop music, meaning the various strains of digitally created music. This is where the money comes in. People that want to make of a living doing music will increasingly need to be proficient in modern music production.
This creates a "virtuous cycle" which directs more resources towards this aspect of music, and a "vicious cycle" towards the traditional aspects of musical performance.
There are actually two significant technological forces that enable this structural shift in music creation. The first was the advent of recording, and mass distribution of music. It broke music away from the need to have human performers, playing in realtime, to hear music. This dramatically lowered the marginal cost of experiencing music.
The twin forces of time shifting and mass replication were turbocharged with the advent of digital audio. This, combined with the ever increased use of digital manipulated in music generation, amounts to a "singularity" in humans relationship with music. A line has been crossed that is permanent, it can never be uncrossed.
To be sure (ha ha), music will continue to be performed (and listened to) by live musicians, indefinitely. But it will be in the context of decreasing cultural influences.
The financial resources needed to support the creation of skilled musicians will continue to dwindle. This effect has been ongoing for decades in the world of orchestral music; now it has come for the world of all performed music.
One might think, what about live music? Won't there always be a demand for live, performed music? I don't think so. Or rather, it will continue the dramatic decline illustrated in the article by guitar sales.
Audiences seeme to respond just as well to shows that use essentially pre-recorded music. As long as there is a show of some kind, most of the music consuming population will not mind if the music heard at a show is "canned".
This makes me a bit sad, but ultimately the endeavors of human creativity will march on, inexorably charting new paths using the astonishing arsenal of software applications that are available these days at a very low cost.