I understand what you are saying, but I still usually don't isolate my fish except to quarantine new ones, or if they are getting bullied, or if a treatment is needed which may harm the others. If one of them is ill, the chances are that whatever bacteria, virus or parasites caused the illness are now in the water, so all the fish have been exposed to them. Some fish react very badly to being put in a small container, and start jumping and splashing around, causing stress and risking injury.

However if a fish needs special medication I still think the best thing is to isolate it.