Basic Dynamic Pressure Equation

P = 0.5 * D * V²

Where:

P: Dynamic Pressure (Pascals or Pounds Per Square Foot [PSF] )
D: Air Density (kg/m³ or Slugs/ft³)
V: Velocity (m/sec or ft/sec)

Types of Egress Methods and their Limitations

References:
Aircraft Design: A Conceptual Approach, Third Edition, Daniel P. Raymer
Pronated Escape System (PRESS), Allen D. Disselkoen and Keith H. Heise

Parachute (Safe Egress below 11 kilopascals / 230 psf Q)

The mass of a parachute system is a few pounds and the method of egress is the pilot either rolls the aircraft inverted, unfastens his seat belt and drops out; or climbs out of the cockpit and jumps. May be impossible for wounded pilots to execute, depending on the severity of the injury.

Ejection Seat (Safe Egress below 56.66 kilopascals / 1,100 psf Q)

The mass of an ejection seat (ACES II) is about 151 pounds (68.49 kg) and the egress method is semi-automatic; the pilot triggers the ejection system and the seat takes it from there. Current “best of” technology is roughly ACES II. Above 56.6 kilopascals with ACES II, the pilot is at severe risk of spinal/visceral injuries due to decelerative forces and flailing injuries due to windblast. There have been proposed prone-pilot ejection seat systems that would enable ejection up to 95.7 kilopascals via the pilot being in a better position to survive the windblast/deceleration, along with strategically placed shields to protect from windblast.

Capsule (Theoretically Unlimited Safe Egress Limit)

The mass of a capsule varies from 500+ pounds for a capsule for a single crew member to 3,000 pounds for a capsule that encloses multiple crew members. (F-111). Egress is semi-automatic, the pilot triggers the system and it goes from there. After separating from the aircraft, the capsule provides complete protection from windblast/cold along with providing shelter on the ground. The F-111 capsule could famously float. There's no theoretical limit to the protection a capsule can provide; it could be used for orbital ejection at any point in the re-entry envelope – it’s just a matter of how much mass the designer is willing to devote to the system.

Commentary

If you play with the calculator, you can see that ejection from an aircraft at Mach 3.0 at 80,000 feet is well within the limits of an ejection seat system (provided that the pilot was wearing a pressure suit), something proven during the SR-71's operational career

So why did the USAF want escape capsules for it’s “next generation” of aircraft of the 1960s (B-70, TFX/F-111) followed by the early 1970s (B-1A)?

I believe it has to do with USAF regulations (then and now) -- a pressure suit is required for operations above 50,000 feet. Wearing a full pressure suit for long periods is extremely tiring as experience with pressure suits on the hot new high performance aircraft of the early to mid 1950s would have showed (Featherweight B-36s, B-47s, B-52s and F-102s). A capsule allows lightweight flight suits to be worn while preserving safety in the high altitude performance regime.