Where do you get 1/3EV gains in high ISO performance? I don't know which cameras you're comparing or what time frame. In my experience, each generation of cameras has offered about one extra stop of high ISO usability. The current 1DX at ISO 102,400 looks about like the original 1D at ISO 3200. That's about 5 stops in 11 years. The 5D > 5D2 > 5D3 progressed about 3 stops in 6 years. That's not marginal to me.
On-chip ADCs sound great. In the meantime, I'm not missing them.
From an actual noise performance standpoint, successive generations of cameras from most manufacturers have not actually realized a true 1-stop improvement. What we have seen is a one-stop increase in the highest user selectable ISO settings, but the performance of that top setting (or even lower settings) has not always improved, from a noise standpoint, by one stop every generation.
Measurable noise from generation after generation of many DSLR cameras from many manufacturers have improved marginally for the most part. The 1D X was one of very few cameras that actually realized a true one-stop advantage over it's predecessor. The 1D IV got about 9.5 stops of DR at ISO 3200, and the 1D X gets about 9.7 stops at ISO 6400. Read noise levels did not change much, but saturation point did as the FWC increased. The 5D III realized no benefit in high ISO DR over the 1D IV or 5D III...less than 1/3rd of a stop at best, and no difference in read noise.
That has been the case for a while, and is generally expected barring some kind of significant technological improvement. High ISO is ultimately physics bound, and as Canon's sensor tech hasn't changed for a decade or so, one shouldn't expect a real difference in high ISO performance. The A7s is a rather intriguing camera in that it realized more significant gains at even higher ISO settings than the 1D X than the 1D X did itself. The 1D X brought native ISO 51200 to the table, at 6.6 stops of DR...the A7s improved that to 8.8 stops of DR. All while concurrently having quite a bit less low ISO read noise (~21e- vs. ~38e-) and more low ISO DR. I don't suspect another major improvement in high ISO performance will occur until another radical technological improvement is made...such as multi-bucket reads or layered photodiodes or some kind of deep charge photodiode capable of holding more electrons than a normal photodiode is introduced. WHEN such technology is introduced, I'd expect high ISO performance to take off again.
In general, though, for a given technology, all we have really seen over the years is an increase in the highest selectable ISO settings...not an actual concurrent reduction in noise that would make that higher stop of ISO perform as well as a lower stop of a prior generation. Only a couple cameras have actually achieved that lately...the 1D X and A7s, and maybe the D600/700/800 series (although they may have simply started out with around an extra stop relative to Canon sensors at ISO 3200/6400 thanks to using Exmor.)