The attribute "unfair" does not seem suitable here, because when some XDG_* envvars breaks wine and it application, all the "fairness" does not help. There seem to be the following options to provide a solution to bug #56941.
1) One alternative is to make the env space in wine as large as on the host system, but this could have implications on the design of wine and its compatibility to windows.
If this is not a suitable approach, we need to accept that env space in windows (and wine) is limited and much smaller than on the host system, and obviously under some circumstances (see bug #56941) some arbitration need to be applied or the application will break in wine. This leads to the following options:
2) Blacklisting: certain host envvars are not provided within wine. This seems to be partially implemented (QT_*, XDG_SESSION_TYPE, ..), or some heuristical approach as suggested by this patch (e.g. remove the largest envvars). This comes at the cost to maintain the "blacklist" and its heuristics.
3) Whitelisting: only a well-defined list of host envvars are provided to the apps within wine. It comes at the cost to manage that whitelist. That might not be completely trivial as all envvars for all libraries used by wine need to be considered, including MESA_*, LIBGL_*, GL_DEBUG_*, DXVK_*, WINE*, etc.
Therefore, the question does not seem to be about "fairness", but about which approach it the most effective solution. Whitelisting seems to be the most deterministic approach, because it prevents the influence of unrelated host envvars.