While I would have loved to have 30, 40 or 50 Canon megapixels a few years ago, I'm not sure the L lenses available in 2008-ish could adequately or comfortably resolve that much detail.Canon has already updated most of the L-zoom range in preparation for higher megapixels. With the 100-400mm already officially on the, we now only await the ultrawide f/2.8 zoom update. Zooms are more versatile and cater to a bigger market than primes so get priority for R&D.
I suspect Canon is pursuing a reasonable strategy of --
1. get the lenses capable of resolving 50+ mp first,
2. then release cameras with sensors that can deliver them.
Anyway, there is no need to worry. There are already many lenses in the Canon Lineup that can resolve more than 30MP and not all of them are expensive. As an example , the cheapest lens in the Canon range, the 50mm f/1.8 can resolve close to 40MP when shooting at f/5.6. The 40mm Pancake is another good performer on a tight budget. Top-of-the-line lenses like the 300mm f/2.8 and the 600mm f/4 could deliver around 50MP of detail given a 54MP sensor.
This is not simply me pull numbers out my ass, it is based on a work in progress. I won't go into my methodology, but you could get an idea of which lenses are good for high res by looking at APS-C crops at TheDigitalPicture website. The 60D is an 18MP body, which when extrapolated to full frame is 46MP. Lenses with good corner performance on full frame hold up to the the APS-C crops. Lenses with bad Chromatic Aberration and soft corners on full frame are not "future proof" and will likely be replaced in the next couple of years.
That sounds like a good objective assessment of the practical issues. I'm expecting the remaining older L prime lenses (the 35 & 50) to be updated soon. It also tallies with word from a professional photographer that there is a high megapixel camera in the wild being tested, but will not be released until the full range of lenses is ready for it. I'm also hoping for replacements to the non-L 20mm and 50mm lenses amongst others. But as noted, zooms are the bigger market, and what I would choose if I had to shoot professionally under pressure.
A high 50MP sensor would give scope for a crop mode, as found on Nikon cameras. A 1.4x crop would give a 25MP image, and a 2x crop would give a 12MP image. That would be a great way to give extra reach to my 70-300L
What is bound to disappoint is the frame rate on a 50MP camera. There will be just too much data for the electronic bus to allow the 10 frames per second of the 7D2. Yet again, not everyone will be happy! Though in the next few years card and processor speeds will increase to alleviate that limitation of current electronics.