We keep doing incremental improvements on a design from 1970 and making it do things it wasn't designed to do. I love the Sierra to death, but they made some weird decisions with it. Sadly, America hasn't successfully fielded a completely new helicopter design since the Blackhawk (unless you count the Osprey, which they tried to kill too many times to count), and probably won't while I'm still flying.
I'd agree that helo development has stagnated. It's taken until the 53k to get a lot of the technology that has been commonplace in fighters for a generation into helos. Part of that is availability of money and part is why spend the money when it already works? For instance, the Army is still flying the Chinook, which has been around since Vietnam. Same with the H-1, H-46, and H-53. Although the 60 design seems dated, it's at least a generation of RW development ahead of many of its peers.
The Sierra was a compromise of an airframe to begin with that USN got on a steal. And while the Navy has done some different things with it, many of the capabilities are ones that the 60 has been able to do for a long time. USA 60s carried hellfires, rockets, and guns back in the 80s. The 60r is far more capable than the 60b or 60f, both of which were improvements over the h-2 and h-3. About the weirdest thing the Navy has done with the 60 is try and make it tow. But they've done that with almost every other RW airframe as well to include the H-3, H-46, and H-53.
Some of sikorskys foreign customers have even added gun turrets to the airframe. It really shows the versatility of the base 60 design that it can successfully do so many disparate missions.