Is there really? Keyboard issues aside, the current MBP line is one of the most future-proof MacBook in years; mobile CPU power has been on a plateau for years now with small incremental rather than big significant upgrades each year, which in turn means that the hardware will continually last longer and longer in terms of CPU power. There are still people on MBPs from 2011 or earlier saying that their machines work fine, I don't think much will change about that when we view the current MBP line a couple years down the road.
The iMac Pro is on similar grounds, with exceptional components all around for its price tag, and I can easily see that most people buying it now still find it usable and suitable for their needs 5-8 years down the road, probably even longer, unless they really really need that additional bit of power future generations might bring. Everything that an iMac Pro or MacBook Pro is able to do right now in a matter of seconds, it will still be able to do 8 years from now.
If anything, I think the lifespan of the average Apple Product is increasing rather than decreasing, seeing as that's also the tendency in Apple's mobile market. The current iPhones and iPads already have more power than many laptops, which means that the average lifespan for, say, an iPhone X is much higher than the one of an iPhone 6 or 6+ where the limited CPU and GPU power and memory became big issues 2-3 years down the road. Many people who previously bought a new phone steadily all 2 years are now more on a 3-4 year update cycle because we've reached a time where the current hardware easily allows that. If you look back into the early days of iPhones and iPads, people were constantly talking about how their current devices were too slow or had too little RAM for any advanced desktop-grade multitasking features and the like.
