The Scots who crossed from Ireland didn't, as far as is known, bring rapine and pillage, but merely settled and retained their written language. The Pictish population apparently quite peacably absorbed the incomers [as they did the Vikings, in spite of horrific tales to the contrary]and probably adopted the language where the populations became mingled. The Picts had no written language as far as is known, and this leads to the false conclusion that they "died out".