The Business Case for Microsoft Dynamics NAV Upgrades: A Partner Details the Costs, Benefits
There's a big difference between "working fine" and working optimally. Still, a good many Microsoft Dynamics NAV users stick by their out-of-support pre-2013 installs because they don't break down. And with yearly updates, perhaps keeping up with Dynamics NAV releases has simply become a burden they would rather avoid.
The fact that older versions of Dynamics NAV don't fall over is a testimony to how solid the product is. Still, Dynamics NAV 2013 and earlier releases simply lack the capabilities and future opportunities of NAV 2016.