I fully understand what you are saying about diffraction and aware of the calculations behind it.
As far as I am aware, a higher MP body will always give more detail, despite diffraction ( and I have not yet seen an example where that is not the case). In other words, for me, it is not whether I am getting 'the maximum out of the lens' but whether I am getting more detail than I already have. And how much it costs to get that difference.
Admittedly, the R7 you then come into how a camera with higher than 20MP would fit into the model hierarchy and cost structures that Canon are developing. They did it with the 7D but as this is a new product line it remains to be seen.
In theory you are right that a higher MP body should always give more detail, despite diffraction. However, this predicates that the construction of a higher megapixel sensor does not lose detail because of electronic and physical constraints. I had at one stage both the 1.6x crop 32 Mpx 90D and the 1.5x crop 20.9 Mpx Nikon D500 and got better resolution at the higher isos I work at with the Nikon. This is borne out in two reviews in AP.
https://www.amateurphotographer.co.uk/reviews/dslrs/canon-eos-90d-full-review
Canon EOS 90D Review: Resolution
A close inspection of our resolution chart tells us the EOS 90D’s new sensor resolves a very creditable 3,700l/ph at its base sensitivity of ISO 100. Detail remains high at low sensitivity settings; with resolution figures of 3,600l/ph and 3,400l/ph recorded at ISO 200 and ISO 400 respectively. Push higher into the ISO range and you’ll start to observe fine detail being affected by noise. We recorded 3,100l/ph at ISO 3200 and 2,900l/ph at ISO 6400. The sensor can resolve 2,600l/ph at ISO 12,800 and 2,400l/ph at ISO 25,600 before expansion, with detail dropping right off at ISO 51,000 (2,100l/ph).
https://www.amateurphotographer.co.uk/reviews/dslrs/nikon-d500-review
Nikon D500 Review: Resolution
The D500 captures as much from our resolution charts as we could realistically expect from its 20.9MP sensor. At low ISOs in raw it resolves around 3,700l/ph before maze-like aliasing comes into play; the JPEG processing tends to suppress such artefacts at the expense of slightly lower resolution. But what’s more impressive is its high ISO capability, with 3,000l/ph still recorded at ISO 6,400. At the highest standard setting of ISO 51,200, it achieves 2,600l/ph, but past this things go downhill quickly. Even at ISO 204,800, we see around 2,000l/ph, but the higher extended settings are too poor to be worth reproducing here.