But then you also need to look at the effects, and how severe they are at influencing.
Having a lower rank being pulled higher, has the downside that the medium ranks are now having someone that doesnt belong there. At plat/diamond this would be common enough to be a problem.
You realy dont want to drag a silver up to plat as a group. He still has to fight his way up there. And if he is playing with a plat group, that means still having a 50/50 win rate.
But if you have a true silver in the group, the higher ranked ones can either risk some losses themselve (a choise those players make!), or make a new account to play with him and get a more balanced rank aswel (it most likely will still cause the silver to rise, but not as severe as your method would). There is going to be a border where the silver is simply not going to have enough of an effect. But its going to take a far longer time to reach that.
Thats why its better to simply just handle him as if he is on the level of the highest, if as team they still manage a 50/50. Its fine. And i would agree that the silver has to rise in that case.
But then i also would wonder: why isnt he able to do that on silver directly? (and the most likely cause would be: poor draft choises and relying on teammates too much. But thats a very diffirent issue that mostly the healer role suffers from.
Its not as easy as it might look, but i rather see the safe method being used. Since a player that can cooperate well in solo, is far more likely to cooperate well as a team. The other way around the team is far more likely to cover up that players mistakes. And they might cover it up very well on that. Which can cause a player that works well as team, to be horrible in solo.