SU ( Skyteam) and S7 ( Oneworld) are members of international alliances. In order to remain a member in good standing, they both have to adhere is international quality standards in order to remain members in good standing.
As I recall SU was admitted to Skyteam with the condition that they build SVO-Terminal D, and retire al lthe older Soviet aircraft, that could not meet the alliance standards. So all of them are gone, and we have SVO-Terminal D.
The other players can do what ever they want.