I like using the adage of keeping a betta in a "bowl" or "cup" without a filter or heater as this. If someone locked you, wearing shorts and a tank top, in an unheated airplane bathroom whose toilet did not flush and you were never allowed to come out. They opened the door a crack once a day and tossed some food in there for you to eat, but you had to live, eat drink, sleep, and use the bathroom in that small unheated space. You could live fine in there for a while. Then eventually you would get cold and sick from all of the waste building up in there. If you were left in those conditions for a long period of time even if someone let you out and you got to live in the cleanest mansion in the world, the damage would already be done and you would never be the same. Such is the life of most betta fish.