Cost / Benefit ratios are a big part of economics. Historically the computer market has been a bit special because the payoffs of new technology were huge and simultaneously the cost of new technology sharply and steadily declined.
If Apple's customers are not still seeing those big payoffs from new technology then it becomes a game of costs. Changing a product mix every few years has its own costs - as demand for new PCs drops, these costs become harder to justify for an OEM.
It is totally possible that a 5- or 10- year refresh cycle keeps engineering and support costs down enough to be worthwhile if consumers accept the technology meets their needs. New technology will still make it to the end user here, but Apple is not going to do it at any cost - they are masters of profitability.