Sandy Bridge was a really really good release, but it's also not useful to ignore that Intel really did keep improving their product for a while. While their product segmentation (keeping mobile parts pinned at 2 cores, and the quad core limit on desktop) definitely was a form of 'stagnation', Intel made real improvements in their micro architecture, at least up to (and including Skylake).
The real 'lost years' were post Skylake, where we got like 5 generations of Skylake refreshes while Intel couldn't manage to get their new node online.
I know we can point at benchmarks and things like that, all I'm saying is that my personal experience is that I spent a decade trying to find something as good as my top-spec 2012 macbook air, which is pretty depressing. Note obviously I also went via the horribly broken keyboard era of macbook pros. Eventually I settled on a X1 carbon, and then a couple of years later the ARM macbooks came out, but I can't justify buying a new laptop out of cycle if my current one works.
The real 'lost years' were post Skylake, where we got like 5 generations of Skylake refreshes while Intel couldn't manage to get their new node online.
Here's a review of Skylake (6700k) from way back showing IPC improvement from Sandy Bridge to Skylake - https://www.anandtech.com/show/9483/intel-skylake-review-670...