If you have an aperture so small that both a high MP body and a low MP body have diffraction, the high MP body will have no advantage. It doesn't matter how finely the sensor can resolve the blur projected by the lens, its still just blur.
Once again, resolution does not work like this. First, diffraction is not a hard limit. Second, even at extreme apertures where diffraction approximates a hard limit, a higher resolution sensor will still yield superior results. Why? Because the resolution of a system is not the weakest link in the chain. It is computed from a formula using all the links and is always less than the weakest one. Increasing any link results in a higher final resolution, but the final resolution is always less than the weakest component.
Practically speaking the increase may not matter outside of a lab. And for 18 vs. 24 MP I'm guessing it won't matter much outside of a lab test even at f/2.8. But putting a higher resolution component in the chain will never result in worse performance.
I hate the term DLA because it's inaccurate and it conveys the idea of a "hard limit" that goes down as the sensor resolution goes up. It is not at all consistent with the science of optics. And it plays off another issue in general conversation: the portrayal of resolution as a single number. It's not a single number. It's an MTF curve. Your comment "it's sill just blur" illustrates the problem. The 7D is "diffraction limited" at f/6.9. The way you describe this, f/8 produces "blur." The reality is that detail with X contrast at f/6.9 has some value <X contrast at f/8, and can be restored to X contrast with sharpening. I can make f/6.9, f/8, and f/11 24" prints all day long and you won't be able to tell me which is which.
At some point detail is truly lost, i.e. contrast of 0%. But the point is not immediately past the DLA.
While a 24MP sensor with DLA setting in at f6.0 will allow for some lenses to shine at larger apertures, where will this marketing machine stop? If the next round of crop cameras hit 40mp, and then after that 60, will you still be arguing for it to carry on?
I've seen convincing arguments for 100-200 MP FF sensors, assuming technology allows you to hold the line on noise/DR. Why? Because of another point that's not reflected in simple DLA numbers tossed around on the web: the impact of diffraction is different for different wavelengths of light. And if your sensor design + RAW software takes this into consideration, it can maximize the detail recovered. We will eventually see that point in digital camera design.
There comes a point where making EF-S glass good enough to resolve such detail at the large apertures needed to avoid diffraction becomes unaffordable. We're already at the point where the 17-40L and 24-105L cost less than their EF-S counterparts.
Which counterparts are you thinking of?
But for those than want the extra MP so they can resolve more detail, in all but very select circumstances and with all but the very best glass, they'll be very dissapointed.
I'll agree that 18 vs. 24 MP is not a very big deal and is driven by marketing. But sensor resolution is not limited by diffraction in the way you think it is. And we will see even higher resolutions in the future. If a jump could be made today to 35 or 40 MP while holding the line on noise/DR, it would produce observably better prints.