Not really true. Whether a camera is more forgiving of a lens or not is solely dependent on the pixel density. Full-frame camera like a D3X pack much the same pixel density as a 12MP camera like a D90. Furthermore, even if the sensor is out resolving the lens, this doesn't reduce the overall resolution of the image compared with using a lower pixel density sensor. Merely when viewed at 100% the image will be softer, but that is a meaningless comparison. When printed to the same size, it wont matter that the sensor out-resolved the lens.
What you claim with a web-sized image is just absolutely wrong. You are trying to say that a cheap kit lens cannot sufficiently resolve a 0.8-1.0MP photo sized for the web, thus there is no benefit in photographing more than the 1.0MP since the image resolution is the same. Clearly this is false because looking at a 12.0MP image form a crop sensor definitely showers higher captured resolution, and when printed to large sizes will show the higher resolution.
Secondly, you are clearly under estimating the quality of kit lenses. Compare the standard Nikon 18-105 VR kit lens to the super expensive Pro Nikon 24-70 2.8 that costs literally 10X the price and is regarded as the best wide-normal fast pro zoom made by any manufacturer:
18-105 it lens
http://www.slrgear.com/reviews/showproduct.php/product/1221/cat/13
Expensive Pro zoom
http://www.slrgear.com/reviews/showproduct.php/product/1121/cat/13
When both lenses are shot wide open there is no observable sharpness difference. Even with both lenses stopped down to 5.6 throughout the zoom range there is barely a difference, nothign you can see unless you are using profesionnal measuring equipment.
And with respect to your previous thread, look how soft your favorite Canon 85 1.2L is on full frame at wide apertures:
http://www.slrgear.com/reviews/showproduct.php/product/354/cat/10
Even when stopped down it is not out-resolving the 18-105 kit lens significantly.
Clearly, sharpness is not the reason for owning different lenses.
And with regards to having to have everything right, I quote form you
"composition, lighting (not necessarily flashes, just positioning or reflectors), subject matter etc" well, sorry to burst your bubble but you will need all of those things right on full frame with even the best prime lenses ever made.
A fancy Canon 1DMK3 and L glass doesn't make good photographs appear by magic. The most important aspects are skills, techniques, creativity, imagination, ability, experience, knowledge, etc.