u r rite but this rule does not apply to human behavior and religion, in the same way.
i wud suggest u to compare like to like.
man has not become robots over a period of time. if quran ask to cut hand of the thief so should we change the requirement, as 1400 years has been passed. unfortunately we have done it and now we are suffering.
(only ismaili and few other sects have the notion to amend the principle of shariah on the authority of their imam)
however, we can change the modalities for doing things but the principle in its essence will remain the same and should be according to shariah. ok if ppl have issues with khilafat then democracy may be considered as a good alternate but the main point here is that legislation must be in conformity wid shariah.