Anyone who buys a MacBook Pro today, before whenever it is the line gets updated, will be happy with it, which is of course a good thing. (Outside of being hit by hardware issues or something)
When the line is updated, you probably won't notice a performance difference between the current model and whatever the next one is. The time of huge improvements with every processor upgrade has long since past, the RAM speed will not be that much different and it will be using solid state storage at again about the same speed as the current model. This is why people will keep their hardware longer.
A lot of hardware upgrades are now coming from when the hardware no longer supports the latest OS. Sierra will work with MacBooks going back to 2009, and they had upgradable RAM and storage so even those laptops will be able to run the latest OS with performance that is acceptable to most people.
All of this means that Apple is going to have a slower cycle for hardware bumps since the cost of a rapid cycle probably doesn't bring the revenue spike it used to.