I must admit that I hadn't really considered the Picts and the Scots to be distinct, in much the same way I wouldn't think of the Cornish/Welsh as being distinct from the Celtic people who were here before the Romans and Saxons (Sais) arrived. And then there's the question of what is "Roman" anyway, most of the legions were cobbled together from an enormously disparate group of peoples. It seems unlikely that a much larger pre-existing populace would simply disappear or move out, much more likely that they would continue much as before with different overlords (who might be more or less beneficent, who knows?). It is true that Saxons and Vikings did settle more extensively, rather than just a few elites, so the make-up of the eastern parts of Britain (especially England) has a bigger mix of those. But lowland Scotland, if anything, appears to have been extensively settled by peoples originally from Wales, so perhaps the lowland Scots are in fact Welsh? :-)
If that is true, in short the Picts were written out of history for political ends. apparently losing their own language in favour of gaelic. The question I ask - in my southern ignorance - is whether they resisted forcibly, decided to go with the flow and join enthusiastically to take advantage of a better standard of life (?) or reacted with a fatalistic outlook akin to the collapse of the Inca empire. However since these were people who, by all accounts, refused to submit to The Romans wouldn't the former have been more likely?