Why do airliners have "pressure bulkheads"?
What part aft of the bulkhead would leak pressure?


That's a partial misunderstanding of what a bulkhead is there for.

You could build the aft cone section to keep the pressure, but it would be a much heavier solution.

The shape of the final aft section is not well suited to resist pressurization stresses: the best shape is a sphere; the cylinder (with spherical terminations) comes a close second. The conical shape would require serious stiffeners to survive pressurization cycles for the whole life of the aircraft; the bulkhead solves this problem by using a shape that is naturally more resistant to stresses - and thus can be built with less material - leading to less weight, and hence fuel savings (in addition to the increased safety).