As developers (and early-bird beta testers) have been working with the various preview releases of iOS 12 this summer, one of the big changes they've seen are performance improvements, particularly for older and/or lower-powered devices. This bodes well for this fall's updates, given that Apple's track record isn't great when it comes to how older hardware handles new iOS releases.
After ensuring that all recent – and some not-so-recent – hardware can run the latest iOS version, Apple has moved a number of devices off the update train.
The same holds true for macOS Mojave, which is the first macOS release in a while to axe support for some older Macs. (Recent macOS upgrades have tended to keep the same hardware/device requirements as their immediate predecessors.) In the same vein, watchOS also culls support for older devices, in this case the first generation of the wearable.
Planning for the arrival of both iOS 12 and macOS Mojave requires an understanding of how the new operating systems will function on hardware that's already been deployed and is now in use throughout a company. (This is true for both personal and company-owned iPhones, iPads and Macs.) With Apple's focus this year on core issues like performance and stability, there's reason for IT admins and budget decision-makers to be optimistic that most in-use devices can be upgraded, either by employees themselves or as part of a corporate refresh as new hardware is deployed.
Still, there will be cases where a choice will have to be made about whether to upgrade a device, replace it entirely, or maintain it on its current OS release.