Medium, the once-promising platform led by Twitter cofounder Ev Williams, is losing 50 staffers — about a third of its workforce — and will undergo a radical overhaul to fix what the CEO has deemed a “broken” business model — advertising-supported online publishing. Williams touted the company’s successes in a post announcing the layoffs, but said that change was needed to address several major business shortfalls that otherwise would continue to plague it. The company will shut down its New York and Washington D.C. offices.