Sorry, that's totally false.
Go try it yourself. There are A7 / A7R test scene samples at multiple sites.
Furthermore, if you need to upsample, starting with more data means the upsampling process is more accurate, as it has to fabricate LESS artificial data than s 24mp or 20mp or 18mp file.
That's true in so far as there's more real data (lpmm at a given MTF) to begin with.
Hang out on a print forum sometime. For all the bickering we do over pixels, they do 10 times more over paper quality, ink quality, dMax, L*, metamerism, bronzing, ink emulsions, etc. It matters to the people at the lab who do all the printing for...millions of people who order prints from labs.
Any of the current Epson Professional series printers/ink sets on a paper like Hot Press Bright...you're looking at the very top end of what can be laid down on paper today. I won't order laser photo paper prints (Frontier or Noritsu) when I can print or order Epson.
You yourself know it isn't just about looking at two images strait out of camera. The editing latitude differences matter as well, the response to things like NR or white balance or exposure changes that better data with richer information that gives us the ability to reduce noise more effectively with less effort, that all matters. Maybe not to everyone, maybe not even the majority (at the moment), but to a lot of people...to an increasing number of people, IMO.
There's not a significant difference between an A7 and an A7R in these respects.
Again, to separate the print issues from actual RAW fidelity. You call it pixel peeping, I call it latitude of post processing and oversampling. Per your point, remember the A7R shoots really compromised RAWS. It is not the same data from the Nikon's more refined pipeline. I wouldn't really shoot with an A7R personally if I had a D810/800 because sony's RAW file choices crippled the A7R. http://diglloyd.com/blog/2014/20140214_1-SonyA7-artifacts-star-trails.htmlhttp://diglloyd.com/blog/2014/20140212_2-SonyA7-RawDigger-posterization.html
you can find more on the subject online off course.
So while the A7R is a great tool indeed, let's just say it doesn't speak for the state of the art 36+MP sensor class.
I've shot with 20MP canon bodies, then moved to 36MP Nikon bodies, and would certainly consider 50-100MP bodies from whatever company can deliver as much in the RAW file as they can (not the A7R way). Surely sony learned and this will carry forward. The point being not the brand which is a secondary aspect. The point being, other than storage, super sampling and higher resolution is always going to trump lower resolutions regardless of media output for my choices. This is why I'll always prefer 36MP to 22 because there is just no benefit to me in the 22MP file, and far too many drawbacks. This is why I'd prefer 50 to 36MP for the same reason and beyond. Now if I covered sports and say, ended up with 2000 images per hour, that equation would change. And off course these high detail cameras aren't aimed at everybody. I recognize that.
Ultimately as I've said it, if you don't value high MP output you're in two camps:
1) you don't need the latitude or would benefit from over sampling. Other aspects rule more.
2) you could use the gains, but are just holding on to what you have because you want/have to.
#1 will always be the case. #2, IMHO, it is inevitable. I don't see canon/sony/nikon ever making another 20s MP full frame again save for the people in camp #1, which I admit is a market. Their landscape/studio game will be 30 or 40 minimum to be competitive. At the density of even APSC, we'd be in the high 40s to 50s territory by now. In many ways, it is not just inevitable, but overdue.
Many of the fears is storage and processing. However the emergence of high resolution photography will provide the tools. As 36-50-100MP become "standard" surely adobe and others will incorporate oversample in their workflow. As indicated by myself and others, color accuracy, aliasing, bayer interpolation artifacts, all are directly tied to resolution with improvements on the MP count making a positive impact. Therefore it stands to reason PS and LR could very much offer you the option to take the 50MP raw, super sample it down to your workflow target resolution, yielding superior results to low resolution capture. You can then choose to discard, or archive the original RAW and work on the derived file.
This super sample step would go BEYOND mere resizing post raw to RGB conversion. RAW->RGB to low MP is what people currently do to demonstrate benefits of 36 vs 22 and surely will of 50 vs 36 but this is not the best way. Ideally the raw processor would create your say 36MP file from the 50-100MP RAW without having an intermediate RGB image that is then downsized using the various interpolation methods. You'll always work and in your case print from the target resolution of your choice, but you just have a better version of it than if you didn't. Again, all inevitable.
Ultimately I'm not trying to convince people to give up their 20+MP gear. I moved out of that, and I know many will once canon catches up and then, surely everybody will build a shrine around some gear and sing praises to all the things I've mentioned. What I care about the trend, not any specific model or brand. There is a lot more than shooting B+W geometric patterns and whatever DXO computers spit out as a number. So when people get worked out about what their 24-70 can do with a pattern in a high vs low sensor resolution, they are essentially missing the forest for the trees. Some will realize this, some won't. And that's ok. everybody should do what works for them.