STV is like putting a plaster on a gaping wound. It won't make the situation worse but it won't make the situation better.
In simulated voting you still end up swinging between a left-wing party and right-wing party with no subtlety in between. Single transferable vote works best when you have lots of small parties with genuine differences between them. People then have an actual choice on who to vote for. We have lots of small parties but they're all essentially one of the two big parties under a different name.