Some of us live where we oppose all of the current elected officials. In that case, it sounds like opting out is the only effective "vote" we can make in a FPTP system
You can reasonably oppose the officials, sure, but the schools and other services in your area still need funding. Avoiding the census is a vote not to fund them, and unlike a normal vote every single return or lack of it has a direct impact on the level of that funding.
Also the census results are applied for a whole decade during which officials can change and many elections will take place.