The impact factor is a journal metric that rewards citations in the first two years after publication (or five in some flavours). For better or worse, this metric has some importance and thus there is some incentive for journals to increase it. Some journals have been accused of artificially inflating it.
If a preprint of a paper was available a considerable time before publication, this increases the chance for citations within the first two years:
The paper can already be cited shortly after publication as related work, corroborating evidence, etc., whereas without a preprint the citing authors would not even have known about the paper in time.
Work sparked by the paper gets a head start to be finished and published within the two-year span.
Authors that would not have access to the paper otherwise can cite the paper.
Thus, allowing preprints has a positive effect on the impact factor of a journal, the only question is how much. Some other journal metrics are similarly effected, but the impact factor is arguably the most prominent one.
Has the link between allowing preprints and boosting the impact factor (or other journal metrics) ever been prominently made? Alternatively, was it prominently opposed? For example:
Have any journals, editors, or other people in charge stated improving journal metrics as a motivation for allowing preprints?
Have prominent advocates of preprints used this argument?
Is there any evidence that journals allowing preprints benefit from this move in terms of metrics? (Eliminating the effect of other trends and factors of course.)