For what it's worth, I'm not particularly worried. My understanding is that the accident was caused by a combination of poor maintenance practices, inadequate preflight procedures, and possibly poor ground procedures by the pilots that led to them taking off with an inoperative pitot-static system. Modern aircraft can also get altitude data from GPS (as you noted), as well as inertial navigation systems and radio altimeters, and generally have redundant pitot-static systems (at least an alternate static air source) for these kinds of contingencies.
Surveillance radars have narrow beam widths in azimuth, but broad in altitude to allow them to quickly survey around them and get good enough awareness of where everyone is. There's always a trade-off between resolution and volume--some military tactical and approach radars have very precise azimuth and altitude capability, at the cost of scan volume/time. Mode C (and now, arguably, ADS-B) was the solution to this problem, allowing ATC radar to have both good position and altitude precision with an acceptable refresh rate, at a non-prohibitive cost.
You'd be surprised--unless your hypothetical binocular observer was really handy with stadiametric ranging, I imagine they'd be nowhere near a mile. They might be able to get a decent azimuth and elevation on a single aircraft, but that's a much different problem than determining position near-simultaneously for a hundred.
Surveillance radars have narrow beam widths in azimuth, but broad in altitude to allow them to quickly survey around them and get good enough awareness of where everyone is. There's always a trade-off between resolution and volume--some military tactical and approach radars have very precise azimuth and altitude capability, at the cost of scan volume/time. Mode C (and now, arguably, ADS-B) was the solution to this problem, allowing ATC radar to have both good position and altitude precision with an acceptable refresh rate, at a non-prohibitive cost.
You'd be surprised--unless your hypothetical binocular observer was really handy with stadiametric ranging, I imagine they'd be nowhere near a mile. They might be able to get a decent azimuth and elevation on a single aircraft, but that's a much different problem than determining position near-simultaneously for a hundred.