Here is my answer. replacement and substitute, relating to objects or products. Let's say you own a Honda motorcycle, and the wheel falls off. That's not good! You go to the motorcycle shop to buy a new wheel. If they have the exact same Japanese Honda wheel, identical to the original, that is a replacement wheel. But if they have a Chinese wheel that fits your bike, looks almost identical, and saves you money, hey, you buy the substitute wheel.

Replacement means the old one is gone for good. The replacement "replaced," or took the place of the old thing or person, and it is the only one now. Substitute is usually temporary. It fills in the place of the other one while that one is being fixed, or sick, or whatever else. After the thing being "subbed" for is working again, the substitute is done. It is replaced, and is not needed any longer.