Hi @folgert! I know that you know about this (and it’s been discussed to varying extents in the papers you mention), but I figured it may be helpful for newcomers to ‘drift’ to say a few words about Sapir’s (1921) conception of drift (Chapter 7, ‘Language as a historical product: Drift’). Sapir defines drift (in the context of language change) as follows:
“The drift of a language is constituted by the unconscious selection on the part of its speakers of those individual variations that are cumulative in some special direction.” [emphasis mine]
The entire chapter gives away that Sapir really does believe the direction to be special and motivated (i.e. non-stochastic). He also talks about drift in terms of cravings and ‘sneaking’ desires a lot. I suppose you could read the passage below in terms of conformist and non-comformist biases:
“Sometimes we can feel where the drift is taking us even while we struggle against it. Probably the majority of those who read these words feel that it is quite “incorrect” to say “Who did you see?” We readers of many books are still very careful to say “Whom did you see?” but we feel a little uncomfortable (uncomfortably proud, it may be) in the process. We are likely to avoid the locution altogether and to say “Who was it you saw?” conserving literary tradition (the “whom”) with the dignity of silence. […] Even now we may go so far as to say that the majority of us are secretly wishing they could say “Who did you see?” It would be a weight off their unconscious minds if some divine authority, overruling the lifted finger of the pedagogue, gave them carte blanche. But we cannot too frankly anticipate the drift and maintain caste. We must affect ignorance of whither we are going and rest content with our mental conflict—uncomfortable conscious acceptance of the “whom,” unconscious desire for the “who.” Meanwhile we indulge our sneaking desire for the forbidden locution by the use of the “who” in certain twilight cases in which we can cover up our fault by a bit of unconscious special pleading.”
But it goes further than that. There’s a passage at the end of the chapter where he discusses the influx of French-Latinate loanwords in English as a case of ‘drift’, as if for drift to happen a language must be ‘ready’ for it:
“The English vocabulary is a rich medley because each English word wants its own castle. Has English long been peculiarly receptive to foreign words because it craves the staking out of as many word areas as possible, or, conversely, has the mechanical imposition of a flood of French and Latin loan-words, unrooted in our earlier tradition, so dulled our feeling for the possibilities of our native resources that we are allowing these to shrink by default? I suspect that both propositions are true. Each feeds on the other. I do not think it likely, however, that the borrowings in English have been as mechanical and external a process as they are generally represented to have been. There was something about the English drift as early as the period following the Norman Conquest that welcomed the new words. They were a compensation for something that was weakening within.”
My sense is that Sapir doesn’t really distinguish between why we innovate and why an innovative trait spreads (?). In any case, what I find fascinating is that a term such as ‘drift’ has found its way into the discussion of cultural change in a ‘stochastic’ and ‘non-stochastic’ sense. I wonder how that happened.
Sorry if this comment drifted off topic 
