Disclaimer: I work on Chrome/Blink and I've also contributed a (very small) number of patches to libxml/libxslt.
It's not just a matter of replacing the libxslt; libxslt integrates quite closely with libxml2. There's a fair amount of glue to bolt libxml2/libxslt on to Blink (and WebKit); I can't speak for Gecko.
Even when there's no work on new XML/XSLT features, there's a passive cost to just having that glue code around since it adds quirks and special cases that otherwise wouldn't exist.
It's not just a matter of replacing the libxslt; libxslt integrates quite closely with libxml2. There's a fair amount of glue to bolt libxml2/libxslt on to Blink (and WebKit); I can't speak for Gecko.
Even when there's no work on new XML/XSLT features, there's a passive cost to just having that glue code around since it adds quirks and special cases that otherwise wouldn't exist.