You could do the job with less, but you’d grumble about it.

I think the shower in my current house is luxurious because it’s large enough that I can turn around without knocking things off the shower caddy.

I always liked drinking water directly from the tub but in the majority of counties people can’t do that so I consider it a luxury. The fact that I live now in a country where it is not possible amplifies that for me even more.

All wares whose demand increases disproportionally with an increasing income.

Bread for example is not a luxury. If you were to earn twice as much, you wouldn’t buy much more bread than you already do. The same isn’t true for jewelry for example.