RB needs to create a situation where Mercedes are the least used engine on the grid, then RB go to the FIA and say "we don't have an engine, so you need to please force the lowest-supplying engine manufacturer to give us an engine contract next year, and oh that appears to be Mercedes."
This would require RB to get either Williams or Force India to switch to Honda engines, and that in turn would require paying the switching team a monstrous amount of money. Probably easier to simply buy Force India and then sign the team up to a dog of a contract with Honda.