The benefit between 22 and 36mp bodies is negligible when using let's say the Tamron 24-70mm. Once you get the zeiss 135mm APO, the difference is stark.
However, doesn't change my point that even at 36mp it's hard to get the detail out of it and let alone a 50mp sensor in 35mm format.
Funny you should mention that lens, inasmuch as Roger Cicala reports that the Tamron 24-70 on a D800 outperforms in terms of resolution the (superior) Canon 24-70II on a 5DIII, a combination that outperforms the Tamron 24-70 + 5DIII. You can't attach the Canon to a D800, but you can (and people do so) to a Sony A7r, so....
I'm not sure what you really mean by "get the detail out of it". You seem here, and in other posts, to suggest that unless you own lenses that can make the most of (whatever that means) a higher resolution sensor, there's no point in using a camera with a higher resolution sensor (other things - such as noise, dynamic range, etc. - being equal, presumably). I'll leave the science to others, but have you tried this yourself or is your argument based on speculation? I've attached a fairly wide array of lenses to my A7r, mostly Canon EF (including some rather inexpensive ones) but a few others as well, including some fairly elderly inexpensive manual lenses even older than those you sneeringly (or so it seems) invoked in another post, and with only one exception so far they are capable of sharp detailed images when viewed at 100%, probably more so than when I use them on my 5DIII (which is not to say the differences are significant). Is your experience different?
But of course the appeal of a sensor, let alone a camera, isn't just its resolution - it's other aspects of its performance; when Sony (or whoever else) releases a 50mm FF camera the other things mentioned above won't be equal. E.g. there are reasons to like the Sony A7 line independent of sensor resolution - noise, dynamic range, that fact that the sensors are housed in mirrorless bodies to which you can attach just about any lens and which have good EVFs, and so on. Given all the other benefits (they're not for everyone, of course), it's nice to be able to attach EF lenses and obtain images that look at least as good as they do via their native bodies. Whether those lenses "gets the most out of" the sensor doesn't matter much to me, and I suspect it won't to lots of others too; you can always supplement them with lenses that perform better. It would be disappointing if the images looked worse, but no-one has provided any reasons to suppose that they would.
In short, yes. My 50L in particular is a great example, It looked really good on my 5Dc but on the MK3 it was less than stellar wide open and stopped down it was sometimes hard to tell the difference between them. The 135L saw a marginal improvement and the 24L II saw it alittle bit wide open. Stopped down performance 5Dc vs 5D3(ISO 100), There is a gain but I sometimes would edit a 5Dc file that I thought was 5D3 file. In practice, then going to print, it was a wash 90% of the time.
The biggest differences were with the 135L between the two cameras. That lens get much much closer to resolving the full 22mp of the 5D3. (which it never will but it gets closer)
My original point here is sony will sell a 50MP 35mm sensor, now will you ever get close to getting the full 50MP out of it with most of your lenses? Probably not, and the differences in practice will be similar to the current A7R. The only way you'll be able to resolve the majority of those 50MP to Definitely SEE the differences, is with the Otus or other lenses like it. Also don't forget we don't always shoot our lenses at F/8 and have to use wider apertures which makes the difference even less so.
Can sonys native A7 Lenses do the job? I don't believe they will but they'll sell alot of cameras advertising those 50MP.