I completely agree with the thrust of your argument. Unfortunately, politicians are uncannily skilled at writing themselves exceptions from legislation. In the US, they write themselves exceptions to most things they pass, like the healthcare laws, and now one of them is pushing a bill that would give all legislators the same status as federal law enforcers (carry a gun anywhere, etc.)
Given they are the only ones who could begin removing their immunity, I can't say I'm holding my breath.
"Quis custodiet ipsos custodes?"
(Who watches the watchers?)