They haven't stopped, as all major manufacturers some series are better than others, some series are better in the low hp range and not so good when every possible kw of power is squished out of the design. The quest is Perkins good engines is in the same category as how long is a rope. It depends, see people talk highly on Shibaura, lot of those was piece of junk in the eighties, the heads cracked, a lot of engines died at ca 4000h, one of the reason was that Shibaura tryed to compete in the 40 to 60hk tractor marked without designing a more heavy duty engine design. So all makes has those bad designs.