People are free to keep their tanks however they see fit however there is a world of a difference between surviving and thriving. A discus may "survive" being kept in sub optimal conditions but they wont "thrive" . Sooner or latter you will have health issues. .Any organism has a range it can tolerate for its environment... but as you move from the optimal range the organism gets more stressed out, becomes more prone to illness. Longevity decreases.They become sick more often and tend to die sooner than if kept more optimally. Its basic biology. If you want to know what a discus optimal range of temperature is look at the wild populations. Im sure they can survive at temps lower than that but there is a cost to the organism if you exceed the minimal range they need. Generally the range for discus is low to mid 80s.
If the goal is to keep your fish alive a bit "survive" regardless of the stress it puts on them keep them at whatever temp you want and accept what happens. If the goal is to give them the parameters they need to "thrive" try to match as many of their natural habitats parameters as you can. Thats what.good husbandry is all about. Its a responsibility to care for your animals that allows them to thrive...not just survive a bit.
Jmo.
Al