Not arguing, but thinking aloud that 300 should be enough with planning for a smaller package.
Sure, depending what and how you shoot. However, IMHO, more buyers than not will buy the A7/A7R and end up shooting zero. I suspect it will be bought by wealthy hobbyists, most of whom don't shoot much anyway.
I may be wrong, but I'm pretty sure that the c300 limit isn't so much a Sony thing as a mirrorless thing, especially when an EVF is involved as it is here; they simply use more power than dslrs. I don't get much more than that with my Olympus OM-D EM5 (haven't had my new EM1 long enough to find out); the first time I went to Longwood Gardens with it I ran out of power after about three hours and had to switch to a Canon dslr.
As for who's interested in the A7/A7R, I have no idea (landscapers with tripods strike me as a small minority, but who knows?), but - and this doesn't really prove anything of course - I spent a fair amount of time in B&H on Sunday. The place was packed, unsurprisingly, with lots of people milling around the Nikon and (especially) Canon stands and wanting to see the latest Fuji Xs and Olympus OM-D. To my complete surprise, while there were a few people at the Sony stand, the only person I'm aware of who ventured near an A7 was me; the lonely salesman seemed pleased enough to chat even though I didn't have much to say except that I lost interest when I realized they had no IBIS - he seemed sympathetic. I must say, though, that it was quite nice to hold and the EVF looks impressive. And it really is small - displayed among a bunch of small Sony point-and-shoots it made no impression whatsoever (aside from being black). Hard to imagine using anything but the smallest primes on it (without a tripod, at any rate).