It is never economical nor ecological to replace something that works perfectly for energy efficiency alone.
When the furnace breaks, get a new high efficency model by all means, but don't just replace one, it takes about 20 years to pay for itself, that way.
The carbon foot print to manufacture metal, paint,plastic, wiring and all the components are staggering, plus the shipping of these components and shipping of the assembled product to the retailer, then to the consumer should be factors in"just buying new" with an energy star next to it just to feel better. Let's not even bring up the cost of credit many use to finance "new and efficient".
We don't really have a way to put a price tag on the enviromental cost of a product purchase like we do an economic one, nor are products labeled as such(carbon footprint to bring to market) save for EEF ratings.
Seems to me if a new product saves x amount of dollars a year in energy costs, costs y amount of dollars to purchase but costs z in carbon footprint to manufacture and deliver, one could plug in costs to the formula to see a ratio or relationship.
I can't think of single reason to throw something away that works well and purchase new just because of energy savings, unless there is a 100% rebate on the item, if you are really thinking green about things.
Appliances, like life, need to be rode hard till the wheels fall off, then fix em.