Tablets may be the PC industry’s biggest threat right now, but slate sales might not always be as lucrative as they are today. The IDC lowered its estimates for this year’s tablet shipments, and also predicts that the tablet market’s growth rate will shrink to single digits by 2017.According to the research firm’s latest report, tablet shipments are expected to hit 221.3 million units by the end of 2013. This is less than the IDC’s original projection of 227.4 million shipments for the year, but is still 53.5 percent higher than slate shipments in 2012. While shipment growth is expected to slow to 22.2 percent year-over-year in 2014, this growth