You are correct, the idea that one should make no demands of the person representing oneself does make it so that person has no incentive to represent you, since they will be assured a more stable base of power if they appeal to other voters that do not support them unconditionally.

I think the reason people do the "blue no matter who" mindkilling cliché is twofold: They've fallen for the "just this once"/"most important election of all time" shtick that has been dragged out for at least four decades.
Secondly they believe in the personal project of the politician - ie. They think that the base desires of the politician is aligned with their own. This is delusional, unless one loves genocide, fascism and a merciless police state.