It's a good point that you would really need the GM lenses to get the most out of this camera. Of course, that's kind of what Sony is counting on, since this beast is only $3500 at launch. Yeah, that's a lot of money, but not compared to the medium format cameras that have more megapixels.
I'm confident many of the Sigma Art primes will be sharp enough for 61MP.
Aside: Now we have to wonder, with the new body revision....will the fabled a7000 and A7SIII use this body?
I think there's still two more rumored camera releases to come. One being an RX....
The other I'm betting will be the a7000 (or A6) in this new body. An APS-C version of the A9 sensor, 20 fps, 30 fps electronic, 5.76 EVF, dual S2 slots, 4K60 10-bit in-camera.
I don't think we're going to see the A7SIII/IV till Jan 2020. I suspect they are trying to cram a 16MP global shutter sensor into it.