Deep air isn't smart for the inexperienced, but it is manageable ("can be handled") for those who have learned how. If Diver "A" can't handle it (because he hasn't learned how to), he's likely to give the "the proper gas at the proper depth" response.
The human body shows demonstrable signs of performance loss at 50'. So do you use Trimix for a 50' dive? So what's the right mixture for this dive?
The gas chosen has to provide an envelope of safe performance. You dive air at 50' even though your performance is diminished, but you're still safe to dive air. What is "the proper gas" depends upon the individual diver. Also, the "proper gas" isn't always available or cost effective.
Because Diver A gets narced out of shape at 80' doesn't mean Diver B will. I have quite a few hours working deep air past 200'. I'm paid to get the job done, not to be narced and unsafe. If I can do the job safely on air, I use air. The proper gas is dependent on the individual and his/her experience.
