See I'd argue that it isn't about money. It's a matter of wanting things to make life fun/interesting. "Normal middle-class people", whatever those are, are always going to want things/experiences/what-have-you -- it's part of the human condition. Conversation, libraries, art galleries have been around for thousands of years, so that's not really going to change things at all. The internet certainly has changed the pattern of behavior for humans as a whole, and made access to information much less skewed towards the upper classes, but I don't see how that's going to make the "middle class" disappear.

In a sense, I don't think most people's happiness is limited by money. Surely the lack of money can bring unhappiness, but I think generally the middle class is happy, and has been for the recent past (just for the sake of argument, the last 3-4 generations).

Maybe my definition of "middle class" is wrong? To me, the "middle class" encompasses the people who still have to work a job, but are able to pay all of their bills in a timely fashion, save some money, and generally support themselves and their dependents. Lower class would mean people living paycheck-to-paycheck or maybe slightly below: just existing means going backwards. Upper class would mean essentially people whose wealth means they can choose to work or not work, and can essentially afford any needs & wants regardless of hours-worked.

Status Anxiety is what you are looking for

But some of those things are expensive. They are just paid for by someone else. A subtle but important distinction. The funding for the opera for example, a token amount might be ticket sales, but mostly it's the taxes of people who have zero interest in opera. So that needs to be factored into the definition.