With Wandering Oasis feat, a druid becomes immune to environmental heat and cold (up to extreme). But he cannot be protected from incredible heat and cold. Then what happens when the temperature becomes incredible cold? I mean, normally a druid would become fatigued after 2 hours and receive moderate cold damage every minute. But since Wandering Oasis would make him treat mild to extreme cold temperature as normal temperature, he would treat incredible cold as mild cold, right? In other words, he would become fatigued after 4 hours and would receive no cold damage? I ask this because I wander whether Wandering Oasis feat would make a druid survive in the world of Frostpunk or not. As you may know already, in that video game, the temperature starts at -20°C and it drops to -150°C in the end. But if Wandering Oasis makes the druid treat incredible cold (which includes -150°C) as mild cold, then it would be child's play for him to survive a glacial apocalypse or heat apocalypse, right?
You're surrounded by soothing energy. You and allies within 60 feet of you are protected from severe environmental heat and cold. If you're legendary in Survival, you and those allies are also protected from extreme environmental heat and cold.
No; once the temperature drops to -62°C, the ability is useless on its own even at legendary survival. Effectively, it cracks under the strain beyond a certain point. There's nothing about it reducing the severity if the environmental conditions exceed its threshold. (Because, as you note, that would make the limitation pointless.)
Now, I did say "on its own". It's not going to stack with anything that lets you treat environmental conditions as one step better (since it's checking the actual environmental conditions, not how you treat them), but it does mean that the druid only needs to raise ambient temperature up 90°C (to hit -60°C) instead of 140°C (to hit -10°C). It's also probably more comfortable than hitting barely survivable temperatures through traditional means.