Even under canopy the likelihood of being at that altitude long enough to get hypoxic hypoxia is highly unlikely to say the least. Having done “flights” in an altitude chamber, you might feel the effects slightly but I’d imagine the intensity and trauma from the ejection event would likely overwhelm your senses for noticing you’re breathing a little heavier.

At 25k useful consciousness is 3-5 minutes. This doesn’t mean being out cold but you’re likely to be severely impaired (almost drunk like) by 5 minutes. That useful consciousness increases rapidly with even slight descents. 5-10 minutes at 22k, 20-30 minutes at 18k, 30 and up at 15k. Hypoxia due to altitude would be the least of your worries if you ever had to eject at high altitude.

Stagnant hypoxia, literal trauma injuries and the complex timing of not hitting the canopy at 9G’s are more likely why the system is automated. It’s hard to pull a ripcord or any levers if the force of wind on ejection breaks an arm.