Though: we speculate that it may eventually come to affect all searches - as mobile-friendliness becomes more pervasive (and hence as non-mobile-friendliness becomes an outlier), we anticipate that they may prefer a good mobile-friendly result to an equivalent non-mobile friendly result even on the desktop.
The main user-centric argument for this is the increasing amount of multi-screening (i.e. returning to the same page on multiple devices) as well as sharing a link via email or social (where the recipient is highly-likely to open it on mobile at least some of the time).
Though: we speculate that it may eventually come to affect all searches - as mobile-friendliness becomes more pervasive (and hence as non-mobile-friendliness becomes an outlier), we anticipate that they may prefer a good mobile-friendly result to an equivalent non-mobile friendly result even on the desktop.
The main user-centric argument for this is the increasing amount of multi-screening (i.e. returning to the same page on multiple devices) as well as sharing a link via email or social (where the recipient is highly-likely to open it on mobile at least some of the time).