A lens is basically capturing a cone of light from the object and focussing it back to a point. So the wider the cone you capture the more light you have to form the image. The need for this is very apparent when there is not much light, e.g. the Hubble space telescope has an aperture of 2.4 meters and focal length of 57.6 meters (a 57600mm F24).
Bigger lenses will make a better image with the same sensor with the same exposure time because they have
more light.
On the other hand what you are stating is that a
better sensor will make better images with the same lens because it
more accurately captures the same amount of light.
(Note that the usual caveats about imperfections apply. If sensors/lenses are not perfect then comparisons between combinations are not straightforward.)
If you want the best image then you should use the best lenses and sensor. So the requirement at the top level for large lenses will remain.
However if you want images to a certain quality with a restricted budget then a better sensor is more economical than bigger lenses.
Also, as previously mentioned, depth-of-field is important in photography. Lens aperture effects depth of field but must always be considered in the context of equivalence (see
http://www.josephjamesphotography.com/equivalence/). Bigger lenses may be the only way to get the desired image look.
