Single players do not have significant enough influence over individual games to truly make this decision. Most people that throw in order to reliably lower their SR do so as groups. (this is based on LFG information and literally hundreds of people that I have reported).
Just because a player is manipulating their SR, doesn’t mean a player can manipulate their MMR. Since SR is not used for matchmaking, then there is a limit to how much a smurf can manipulate the outcome of individual matches when alone.
The reduction of throwers and boosters is actually one of the primary reasons I advocate for changing Role Queue to a Solo Queue only model. Competitive Suggestion - Solo Queue