My understanding is that for every such flight a tender must be opened, and other local operators have the opportunity to veto this flight.
But as others have suggested in the end it depends all on connections, and rubels, ...
Another advise, you will get a customs letter for the domestic sectors. You MUST adhere to the route stated thereon (in kyrillic), or must have customs revise that letter BEFORE departing for a flight different from the original routing.
Non-adherence can result in a severe hiccup...