Not pressurizing the plane at all is possible (the bombers of WW II were unpressurized for example until the B-29) but is probably not a good idea in light of t

Not pressurizing the plane at all is possible (the bombers of WW II were unpressurized for example until the B-29) but is probably not a good idea in light of the fact that even the F-35, where weight is very costly, has a pressurized cabin even though there is only ever one person in the cabin. They wouldn't have done that unless the need for pressurization was great: weight is very costly on the F-35. For example, they did a lot of research and development to design an intake with exactly the right shape to avoid the need for a variable intake ramp (and the actuators needed to vary the position ramp). For another example, they developed the software necessary for the plane to do "aerodynamic braking" to slow the plane down after touchdown, which eliminated the need for a parachute (which a lot of older fighters had) and reduced the need for wheel brakes (allowing the landing gear to be lighter). Reading more, the F-35 is designed to "operate above 50,000 ft, where outside pressure is near-vacuum" (quoting an LLM). The un-pressurized bombers of WW II couldn't operate at those altitudes (even though the crew wore heated clothing and breathed supplemental oxygen delivered through masks).