28nm was quite a cheap fabrication technology even in 2015, but it costs a lot to have a completely custom production run. My guess it that it approximately works out in savings of power and space over the lifetime of the chip. It probably doesn't make sense for them to move to something smaller (and thus more expensive) whilst the performance benefit remains so substantial. If I were Intel, I probably wouldn't lose too much sleep over it either, because you still need something to attach the highly-specialised TPU to, and that'll be a Xeon for the forseeable future.