The Cooler Danchou
Question that is most important for me is: Does this signal the end of the era, in the same way Legendary Treasures did?
No. The Japanese version of Legendary Treasures (EX Battle Boost) was a regularly scheduled set that fell after the last BW set but before XY1. The Premium Champion Set is a "Concept Pack" (like Magma vs. Aqua or Shiny Collection) and we know that the XY BREAK block is continuing afterward because XY11 has already been announced and will be coming out in June, scheduled just like normal.