Many crises are preventable, and this is one of them. When you decide to keep old equipment until it fails, the failure will happen at a time of the equipment’s choosing, not a time of your choosing. This leaves you scrambling to replace equipment in a hurry, with little time to evaluate alternatives, and delaying other work.
Equipment should have a mandatory retirement age. This allows you to plan (and budget for) hardware replacement. Synchronizing equipment retirement with warranty expiry dates will ensure that equipment in service is always under warranty. This arrangement also works well with equipment leasing, if you get 36-month leases and 36-month warranties.