Back when I first started hearing the "alt coins drop in price when BTC rises" stuff, it was after the first BTC pump. People would take money out of ALTs to put into BTC. A year after BTC reached it's first all time high, it was down around the 220 mark and stayed around that level (220-300 or so) for pretty much all of 2015. I think people learned their lesson that diversifying is a far better idea.