With the current state of the Canon lens line up I think that an 18 MP camera with much better image quality in a 5D III is the best solution.
It depends on what you expect. A 7D shows how the center portion will look of a 46 megapixel full-frame sensor. You will see dropoff in the corners, especially at large apertures of course, but at say f/8 you'll see good performance from many current lenses.
Here's a few example of current lenses that I think would work well with a high resolution sensor (in the 36 - 48 mp range):
TS-E 17, TS-E 24 II, 50/1.4, 85/1.8, TS-E 90, 100/2, 135/2, 70-200/2.8 II, 400/5.6, + high end tele lenses. These will provide good corner-to-corner performance at f/8, noticably less sharp when pixel-peeping yes, but no extreme differences. In short DoF photography corner performance is usually not very important. At 36 megapixels the pixel pitch is only 25% smaller than current 5Dmk2, so it is not extreme resolution increase we are talking about. If lenses were really bad at 36 megapixels, it would clearly show already at 21.
Wide angle and standard zooms will probably show the weakest performance (70-200 is a much better performing zoom range), and won't be the choice for a high res photographer. On the wide end we have the new TS-Es which are great, but I'd like to see a new TS-E 45mm, the current is not too good from a high res perspective.
Getting the most out of high res sensor does require expensive lenses on the wide end and on the tele end, but in the range 50 - 100 sharp is cheap.
It is not necessary to limit the sensor to what the worst lenses can provide. There will be binning modes, sRAW etc, for hand-held, high ISO, and less sharp lenses, you won't have to use max resolution when you don't need it.
There's also another aspect - "sharp pixels" (=aliased, jaggies) don't enlarge well. Is is good if the sensor has so high resolution that you get fairly soft pixels which can be stretched around in PP and enlarging without showing any artifacts. Low-res images without AA filter (i e Sigma) is the worst for enlarging.