I will try to answer this question by the bullet point you ask
> Better paying customers can fly more reliable/newer/well maintained planes.
The plane at the current status is as well maintained as it could be. You can better maintain the plane to increase reliability but at a huge cost with little improvement. Is that worth it? No.
> Economy class can mean economy for company too. Economy class will enjoy accessible prices by flying less maintained plane or plane with one engine not working. And maintenance is expensive.
Why don't 747s use unreliable, but cheap, jet engines instead?
In short cheaper stuff gets you no where. Twin engine plane with one not working is going no where. Plane that often breakdown goes no where. It will incur more cost to the airline to rebook passengers to another flight, provide them meal and accommodations as needed. And would anyone book a flight that has breakdown reputation? In the long run it is cheaper to have a reliable plane that depart on time and does not breakdown in flight.
> More space for luxury, e.g pool, billiard tables, spa, jacuzzi, casin.
Do they want that kind of luxury? 747 in the past had that luxury but it was removed? A380 had on board lounge in some configuration but not all. Some planes offer shower on board but not all. Why? Because this kind of luxury cost more than it is worth. A single person jacuzzi I found online has capacity of 180 litre which means 180kg of water alone. That is the weight of 2 passengers. You can call that the cost of luxury but do you want to pay for the cost of 2 passengers just for the water? I’m not sure.
• Safer travel, better paying client can have stuff that is normally not considered bacause of high cost, eg individual parachute per passenger.
Why are commercial flights not equipped with parachutes for the passengers?
Parachute is not a magic wand that you just whip and it will work. You need proper training to wear it properly. You need the plane to fly smooth enough to jump out of. You risk breaking your leg when you land. And worst of all you are paying for high cost item that you are not likely to use since you have an already well maintained plane.
• Super economy class, ultra cheap, no frills plane, passengers and baggage share one big compartment, baggage is fastened to floor with rope, while passengers have to clutch some kind of handrail (not unlike bus) during landing/takeoff.
You need to certify this with civil aviation authority. Good luck with showing them how can this be safe for all age group and sex.
• Seats take significant weight that could be used for extra passengers.
• Novelty and dangers of super cheap flight could attract thrill seeking customers.
Dangers bit caught me immediately. Here we are providing air travel service. We provide safety. If there is any bit of danger the airline will try to improve it, I hope. If you want danger please do it on the ground or in aerobatic plane.
• Seatless planes could have novel or extreme onboard entertainment. E.g is cock fighting or dog pitting still illegal over international waters ? Passengers could make bets. What about underground boxing ? Winner gets first class voucher.
I’m quite sure cock fighting or such is not illegal over international water but the plane is bound to the law of country of registration no matter where they are. So unless you can find a way so that passenger can be outside of the plane to bet on such this you can bet this to be illegal. And if this can be done what are you trying to operate? A flying casino? Good try man.
• Customised worst case gurantees, e.g if you crash we promise to look for you X days.
This kind of guarantees is already provided in the current contract of carriage. In summary the airlines already promise to fly you from one place to another on the date and time you book for. They will do their best if there is unfortunate event occur. Usually if the plane crash they will cover the cost of treatment and if they fail to do it is a very bad airline. Will you trust to fly with airline that crash and make you pay the hospital bill?
In short it may not worth it to separate plane in to different class to justify the cost of operating it when you can have a different class on the same plane and operate it efficiently. There is an example of Singapore airline that operate an all-business class plane for Singapore-New York route but that is because the plane need to be lighter to carry more fuel for the route so they cannot carry all economy passengers. Other than that I cannot find the reason why would you want to operate the plane the way you explain.
Why 1st and 2nd class but not fast or slow airplanes?