the most economical time to overhaul the engine would be at the manufactures TBO (or a little over depending on the type of operation), the further you run the engine obviously there will be more wear on the various internal components.
If a crankshaft that would have been within limits at say 2000-2200 hrs needed to be replaced because you ran the engine to 2400-2500 Hrs then that 200hrs could have easily cost you an extra $10k
The other thing to think of is at overhaul parts are subject to all types of non destructive testing (magnaflux, fluro penetrant, ultrasonic) to check for defects, cracks etc. i seriously doubt that the manufacturers invented these checks just so that they could sell more parts
Whilst an engine might be running great at 2500Hrs might have good compression, oil pressure, develop power etc does your LAME have a crystal ball to look into that will tell him that the No. 3 Conecting rod bolt has a microscopic fatigue crack that is about to grow out of controll and spoil your day ?