I don't think it needs to be a rigid update every year but they should have a policy of not staying more than one generation behind. The 5K iMac jumped to Skylake, so that's fine, but the rest of the iMac line is stuck on Broadwell, the Mac Mini is Haswell, and the Mac Pro is Ivy Bridge.

The solid industrial design, long service life, etc. are worth something but probably not 3-4 hardware generations, especially for the premium market Apple wants to be in. The PC market hasn't been completely stagnant and while nobody is buying a Mac for super hard-core gaming they really don't want to be in a position where someone drops a fair amount of cash and their new computer struggles with a game which came out a year or two back, 4K video playback, etc.