I've actually done extensive testing to determine the resolution capabilities of Canon's current lens line up.
Most of their lenses are only capable of about 14 megapixels of resolution. Their L lenses tend to be closer to 17 and handful of lenses such as the 70-200mm F4 IS can go all the way up to 36 megapixels based on extrapolating results from crop cameras.
Here's the weird thing about resolution though, just because a lens can only render 14 megapixels, doesn't mean it needs a 14 mp sensor.
The best way to illustrate this is to go into photoshop create an alternating black and white set of lines. Then resize it using nearest neighbor to 10x it's size. Offset it by just 2 or 3 pixels and then resize it using bicubic to the original size.
What you did there is took an image of a certain resolution and did the equivalent of trying to render it at it's own resolution while inserting random variation. In the real world a len's image does not line up perfectly with a sensor's pixels. Lens tests compensate for this by measuring at hundreds of points. The reality is though that if you had 1 pixels for every point a lens could render you'd only capture 50% of the len's resolution.
Thankfully we have sharpening algorithms which help to even out the issue, which theoretically can halve the issue, unfortunately cameras use bayer sensors which make the problem slightly worse. In the end if you had 1 pixel for every point a lens could render on our cameras you'd only capture around 70% of it's resolution after processing.
The theoretical point of deminishing returns when trying to render a lens follows this graph, as mentioned earlier sharpening algorithms make the problem about half as bad, so this assumes there is zero sharpening:
Theoretically if you used a 14 megapixel lens on a 14 mp sensor and compared it to the same lens on a 28 mp sensor (double the resolution of the lens) you'd see a 21% increase in resolution, a 56 mp sensor (quardruple) would result in a 32% increase in resolution over 14 mp, and a 112 mp sensor would get you a 37% increase.
So there is some benefit to having way more resolution than your lens can render simply to catch all the contrasting edges which do not perfectly align with the pixels on your camera.