I don't want an argument, just making that clear now. 
As far as bettas in cups go, sure its doable, I've even done it myself when I first started. But after keeping my betta in a 120ltr tank for three years and seeing the difference in the quality of life he has, and how active he is with all that room, I'll never personally keep a betta in anything smaller than 30ltrs.
When I googled the lifespan of bettas I found that most live for two-three years. Bruce (my betta) is over 3 years old and still acts like he has years of life in him. Personally I credit his health to his diet, water quality and --- tank size.
Also, does anyone understand how muscle deterioration works? And does it have the same principals and effects on fish as it does on mammals, reptiles and birds? Because if it does..

As far as bettas in cups go, sure its doable, I've even done it myself when I first started. But after keeping my betta in a 120ltr tank for three years and seeing the difference in the quality of life he has, and how active he is with all that room, I'll never personally keep a betta in anything smaller than 30ltrs.
When I googled the lifespan of bettas I found that most live for two-three years. Bruce (my betta) is over 3 years old and still acts like he has years of life in him. Personally I credit his health to his diet, water quality and --- tank size.
Also, does anyone understand how muscle deterioration works? And does it have the same principals and effects on fish as it does on mammals, reptiles and birds? Because if it does..