In order to have secure systems, you need to add a lot of complexity overhead that earlier systems simply could not afford. Proper process isolation is a good example - extra complexity, but absolutely necessary unless your chip limitations make it impractical. Permissions for memory pages eg. W^X policy. Stack canaries. Address space layout randomization. All things that add extra complexity and resource usage, but without which it's much easier to exploit systems and any single mistake means it's game over.