Comma after last author field in biblatex

Just in case someone is still interested in this in 2018.

This is now easily possible using biblatex's context-sensitive delimiters with

\DeclareDelimFormat[bib,biblist]{nameyeardelim}{\addcomma\space}

A possible solution is use the xpatch package to patch the date+extrayear bibmacro as follows:

\usepackage{xpatch}
\xpretobibmacro{date+extrayear}{\unspace\addcomma\addspace}{}{}

EDIT: Here is an explanation of the patch. The biblatex documentation contains a bib latex.def file with definitions of many bibmacro and other aspects, covering among other which and when bib fields are printed and formatting instructions to print them. Then biblatex include standards.blx. In this file there are the definitions of the drivers. The definition of the drivers specify the order of the various elements. The date/year does not appear directly in the main drivers, but it appears in biblatex macros. For example, for the authoryear style it appears in the macro that produce the author(s)/editor(s) (and the authoryear-icomp calls the authoryear style (for references).