That's like saying one person taking their bike to work instead of their car won't do anything, as if their car doesn't burn gas, as if they don't give money to oil companies for that gas.

The same idea with private jets. No matter how small a positive change might be in the grand scope of things, it's still a worthwhile change. That's really the one thing that has to happen so we all don't die; everyone making pro-environmental life choices that will eventually carry over into industry when that collective pressure accumulates, forcing companies that rely on income from those people to change their practice and policy.