This is not true for the average person in the UK and the developed world. Real wages have been essentially flat from the late 70s until 10 years ago when they started to go down. Some things got cheaper like electrical gizmos but more important things like housing, education and the like have tended to get more expensive in real terms. The top 1% have done unhealthily well, those that have been able to obtain assets like a house have done OK (essentially old people) but those without significant assets are not doing well and they are going to do even less well in the future if things continue as they are.
You touch on one other aspect here, younger people are more likely to rent and thus be more mobile; there's pressure on space in the cities, and despite we're still urbanising at a hell of a rate. I've just turned 40 - I only bought my first hifi system in my late 20s, once I had done with university, travelling, etc, and despite not being particularly flush and still renting, had enough stability (and a living room suitable for parties) to warrant a more serious sound system! Before then, it was not even in question, due to the hassle of frequent moves, flatshares etc., despite being aware of the merits of a good system (I play guitar, had been gigging, my folks had a half decent system at home, best mate's dad was seriously into hifi, etc). The situation has only got "worse" - the result is that people have less space and less inclination to buy bulky stuff that needs to be carefully moved, because the space they're living in is shrinking and they're moving more often. It's not necessarily that they're not financially able to buy, but there are other constraints in play. In this context, a single box system makes a lot of sense, e.g. either Sonos and their ilk, or something like a Cyrus One and a pair of small speakers. And, of course, the headphone scene, which is a great stepping stone. Some manufacturers have cottoned on to this, but many haven't.