For me, bicycles have to be the complete package of function and form and in many cases it is just swapping out the component that does not provide the function you want or the form which could be as simple as matching the bar tape and saddle or changing a component that does not match up.

A modern derailleur or crank might offer all the performance one wants but look very out of place on a vintage bicycle and there are period correct components that don;t give up much, if anything and just look right.