IBM learnt from these mistakes by providing entry points to their ROM
BIOS in another way: instead of using fixed addresses, they reserved
16 of the 256 possible interrupts as documented entry points for the
ROM BIOS. Early PC's came with as much technical documentation as
the Apple II did, including full schematics and assembly source code
listing of the ROM BIOS.
Using soft interrupts as entry points on the Apple II would have been
infeasible -- but Apple could have used a JMP table instead, positioned
near the beginning or the end of the ROM address space.