I can't see it as destroying anything. It's hard to understate the technological impact this new technology is going to have and we've really had less than a generation to deal with it. So, things should seem pretty chaotic.
As far as our economic status, for, one, the internet has made a hell of a lot more jobs than it has "destroyed". There are entirely new industries, mostly outside of basic IT, that have sprung up as a result. From graphic design and SEO to programmers and customer support. So, I can't see the Internet "reducing" the number of total available jobs. There are plenty of statistics that support the statement that there are a lot of unfilled jobs out there that don't have enough applicants due to the inefficiency of our education system to adapt to the new work force expectations. But, I think it'll balance out in time.
As sad as I am to see "brick and mortars" going away, most of my concern has been that loss of the social aspect that they provided. I have to nostalgia for the loss of service. Right now, I buy most of my entertainment directly from the creators or the distributors. And I think that's a good thing. It removes what from an economical standpoint was an unnecessary "middle-man" that simply added to the final costs. I used to spend a lot of time, quite literally, traveling around the country, hunting down obscure records stores and paying relatively huge sums to buy obscure releases from obscure artists. Now, I pay less money and I pay it directly to the artists. And, I think that's a lot better both for me as a consumer and the artists as creators.
The guitar example is a good one though, and I'll agree that there are some products that really do benefit from being able to test them first. But, I also think that a lot of internet-based sales companies have adapted to that with a very gracious return policy. For example, I have never had any problems returning opened video cards to Microcenter just because they ended up not meeting my specific needs once I had installed them in my systems. It used to be that there were no returns once you opened them. But, Microcenter, for whatever behind the scenes reasoning they had, relaxed that policy, and now I know that I can impulse buy from them with little-to-no risk.
As for the loss of manufacturing, well, I think that's just a sad side effect of globalization and a natural effect of capitalization more than anything else. I don't think it's anything that will really ever come back to America. Our cost of living is too high compared to the rest of the world and that's just something that we're going to have to accept and adapt to. But, I also don't think it's really going to harm the country as a whole. It's just going to change things. This country has always prospered primarily as an innovator, not as a producer, as far as the global economy is concerned. And, I think that is something that we can continue to encourage.
As far as the working classes go, what I suspect we will see is a shift from manufacturing and selling to a service based industry. We may not build the cars, but we'll always need mechanics. And, I think that analogy can be stretched to just about every industry in one way or another. Particularly as complex technology becomes more integrated into our daily lives, we'll always need skilled technicians to repair and modify the technologies that we are increasingly depending on. But, it's up to the working classes (and all of us, really) to adapt to these new needs.