I suppose I'd have to disagree with that last statement, as someone who lives in a functioning multi-generational household lol. It's not going to work for everyone every time, depending on circumstances, but having 3 generations in the house pooling resources is how most of the world does it.

It certainly helps to have a separate space/entrance for privacy and such, but now that we're settled in, it's far better than it was when we were going it alone. We have everything covered between the 3 adults, and then some: I work, cover utilities, build stuff, cook repair and maintain everything that needs repair and maintenance. The wife stays at home with our toddler. Her mom covers the mortgage and occasionally babysits when we need it. The occasional family friend who needs a place to crash for a while contributes their bit. We make adjustments here and there to keep everyone happy and maintain fairness. Everyone wins in this arrangement, and there's no shame.

Your example is kind of an extreme trope IMO, I've only seen true "failure to launch" scenarios in media. I'm sure they exist somewhere, but it's not too big an issue to be moralizing about.