How long a socket is around isn't really relevant at all these days, same-socket CPU upgrades haven't made any sense on Intel for more than a decade.
The very simple fact is that a 5820k has 6 cores and a 4 channel memory bus, while a 6700k has 4 cores and a 2 channel memory bus. If you subscribe to the prediction (which is pretty well-founded I'd say) that applications will get increasingly well parallelized in the future, then having more cores and significantly more memory bandwidth is better than having fewer cores and less memory bandwidth.
The very simple fact is that a 5820k has 6 cores and a 4 channel memory bus, while a 6700k has 4 cores and a 2 channel memory bus. If you subscribe to the prediction (which is pretty well-founded I'd say) that applications will get increasingly well parallelized in the future, then having more cores and significantly more memory bandwidth is better than having fewer cores and less memory bandwidth.