Sales of HDDs have been dropping for years now, in part due to the weak PC market, but HDD makers continue to innovate with new technologies to increase areal density. AnandTech published a nice article detailing the emerging technologiesfor capacity growth:
In addition to two dimensional magnetic recording and heat-assisted magnetic recording technologies — that Seagate expects in commercial products in 2016 – 2017 and beyond — other technologies are being researched by the industry. Among those already disclosed are Bit Patterned Media Recording (BPMR), Heated Dot Magnetic Recording (HDMR), Microwave-Assisted Magnetic Recording (MAMR) and some other. In the future, hard disk drives will adopt combinations of various technologies to maximize bit densities and capacities.