You don't live in an optimal world so what use is it talking about? Are you also doing your business calculations using frictionless spherical cows in a vacuum?

You don't live in an optimal world so what use is it talking about? Are you also doing your business calculations using frictionless spherical cows in a vacuum? Simply put every living system on the planet earth has some amount of parasites. To have no parasites at all would require massive amounts of energy by the host to ensure said parasites don't exist. If the host can spend a much smaller amount of energy and ensure that 99% of parasites don't exist that is optimal thereby negating your original premise.