Assuming there are no arbitrage opportunities, the price of a derivative depends on the prices of other derivatives available in the market. If you introduce a derivative without giving it a price, or by giving it a price that is already theoretically determined by your initial setup, then you do not affect the prices of other derivatives.
To take a simpler example, suppose we have two sources of risk, two (biased) coin tosses $\omega_1$ and $\omega_2$ and the only tradable asset is $S=\\\$1_{H(\omega_1)}$ i.e. we get 1 dollar if the first coin toss is heads and 0 if it is tails.
Now we can complete the market by introducing an option that has payoff $$ V=h \\\$1_{H(\omega_1)} +\\\$1_{H(\omega_2)} $$ where $h$ is a huge number.
Assume there are no arbitrage opportunities and the price of the option $V$ is $\\\$1$. Then $H(\omega_1)$ must be very unlikely and so the original asset $S$ is now essentially worthless. But if we don't assume the price of the introduced option to be given then introducing the option has no effect.