The problem with your idea is the part about eliminating the bathroom. For a household with that many people, having a single bathroom is really a big PITA. As soon as one person is taking a shower, that means everyone else has to wait 15-30 minutes for them to finish before they can use the toilet. It's even worse for guests. Bathrooms don't have to take up that much room; you can stick them in a space about the size of two old-style telephone booths, for a simple half-bath meant for guest and emergency usage only. These days, no one wants to buy a house with a single bathroom for these reasons, and in older houses people will even turn a closet into a second bathroom.
The idea behind "open plan" spaces is to make it seem more roomy by combining rooms and not having so many walls. People are both larger and taller than they used to be, so tiny houses can seem claustrophobic. Combining your living areas (living room, dining room, kitchen) into a single space actually makes sense here. Americans don't even use formal dining rooms any more, so it's a waste of space to have them, so now they're either eliminated altogether or made part of the living room or kitchen (the so-called "eat-in kitchen").
Now, as for Americans' "obsession" with open plan, this does not apply to offices; the dynamics behind house design and office design are completely unrelated. For houses, it's because people want a more spacious feeling, like I said above. For offices, it's because corporations are cheap bastards who want to save money on office space for their office drones, so they've made up a bunch of nice-sounding BS about "collaboration", when the real motivation is to greatly shrink the amount of square footage per desk because the open-plan concept lets you shove more desks together more closely, and commercial real estate is expensive. There's also a certain amount of managers liking to be able to see what their employees are doing. In reality, open-plan offices absolutely kill productivity for workers who need to concentrate on their work, such as software developers.
The idea behind "open plan" spaces is to make it seem more roomy by combining rooms and not having so many walls. People are both larger and taller than they used to be, so tiny houses can seem claustrophobic. Combining your living areas (living room, dining room, kitchen) into a single space actually makes sense here. Americans don't even use formal dining rooms any more, so it's a waste of space to have them, so now they're either eliminated altogether or made part of the living room or kitchen (the so-called "eat-in kitchen").
Now, as for Americans' "obsession" with open plan, this does not apply to offices; the dynamics behind house design and office design are completely unrelated. For houses, it's because people want a more spacious feeling, like I said above. For offices, it's because corporations are cheap bastards who want to save money on office space for their office drones, so they've made up a bunch of nice-sounding BS about "collaboration", when the real motivation is to greatly shrink the amount of square footage per desk because the open-plan concept lets you shove more desks together more closely, and commercial real estate is expensive. There's also a certain amount of managers liking to be able to see what their employees are doing. In reality, open-plan offices absolutely kill productivity for workers who need to concentrate on their work, such as software developers.