Microsoft normally supports products for 5 years or more, don't they? So you're saying you want your estate to stagnate for 5 years? An estate that remains untouched for 5 years is not a stable estate. New software products that your users might need come out in that time, and they will have compatibility requirements of their own (because the vendor will have only tested against modern OS releases). And because it's been untouched for so long, there is nobody in the company that actually knows how to update the estate (there are no practices, tools, procedures), and the users are used to their machines never being touched, so it would be super traumatic for them when you did do it, and then on top of all that, when you were finally forced to update things (at which point this has become bigger than Ben Hur), the change would be so big because you'd left it for so long, that it would be a shock to the employees and error-prone to deploy. Frequent small changes are far better than infrequent big changes, not only because there is less change in between updates, but because by doing more of them, you get better at testing and deploying them.