Would be interresting which lenses meet this resolution at an high image quality.
In Spring I visited an collegue, who works for an big optical glass producing company in Germany and I was able to look at some - non confidental - production processes.
He is specialized in the production of special glass for astronomical lenses. Maybe for some military products too...
He told me that it would only make sense, if they use more highend glass for highly priced lenses to minimize CA and other optical problems. But then one lens would cost 2-5 times the price of an existing lens.
This could be an crux too. If this is neccessary, the expensive lenses will get astronomically high priced and the normal enthusiast will be only able to buy mid-ranged lenses (where Canon will not put all its efforts in developing an extraordinary IQ in an mid ranged lens)
I think Canon's latest generation of lenses, the ones that have been getting released over the last few years (most of the Mark II generation, with the exception of the ultra-wide angle stuff like the 16-35 II), is probably more than capable enough for 75mp worth of pixels in a bayer type sensor. I would say they are probably good enough until 35mm pushes into the hundred megapixel range or farther.
If we assume that at some point, FF and APS-C sensors will use the kind of small pixels we find in the most recent phone and P&S cams, which is around 1.2µm on a 65nm BI process, then we would be looking at 600mp FF (30,000x20,000 pixels exactly) and 230.75mp APS-C (18583x12417 pixels). It is easier to optimize a lens that is small, which is why we don't see severe optical aberrations in smartphone camera photos. There would certainly be some challenge in optimizing lenses to support 600 megapixels of full-frame goodness!
I agree that consumer-grade lenses will suffer, and won't necessarily be up to snuff to extract the most from a high resolution sensor. Keep in mind, though, final image output resolution is a convolution of the resolution of the lens and the resolution of the sensor. Increasing either will increase the resolution of the final output, so it is not like a poorer grade consumer lens will really drag IQ down...you just won't get as much out of the whole setup as if you had a high end L-series lens. The same also goes for say slapping one of today's top-end L-series lenses on a hypothetical 600mp FF camera...you would definitely see an improvement over slapping one of today's top-end L-series lenses on a 40, 50, or 75mp FF camera, even if it isn't ideally optimized for the higher resolution.