You CAN carry as support. I do it all the time. Get vision, push objectives, direct your team (non-stop). People will accuse you of flaming or toxic if you're constantly saying things like "do dragon....do dragon...do dragon...do dragon." Problem is, it's 100% not flaming/toxic to remind someone after they get a kill to take an objective, especially at low elo.
With that said, this IS 100% the problem. MMR is broken. I will accept that I will probably continue to need to remind people to take objectives forever, but to be put on a team with someone who doesn't understand the game after winning 7 games in a row is a clear sign that the MMR system is broken.
I suspect (with evidence, anecdotal perhaps) that the system places people on the climb with people on the fall to "balance" teams. So someone who was 900 and winning might be at 1000, and someone who was 1100 and is losing might be at 1000. Those are NOT the same quality of players. Yes, the numbers don't mean anything here, just showing the obvious difference in a clear way.