We believe NFTs should need to be renewed over time to incentivize players to get rid of their NFTs regularly. This will also drive up the value of old NFTs because they will get rarer over time, as players try to keep up with the rate of emission of new NFTs.
For example, units which were minted at the start of the game (generation 1) will be weaker than units minted months later (generation n). However, players who hold units from generation 1 will be able to fuse their units and try to get units from generation 2 and up.
Fusing and upgrading NFTs will burn the lower tier ones, allowing players to upgrade their NFTs and increasing the value of older NFTs for collectors. Fusing NFTs also comes at a risk due to the possibility of failure which will furthermore deflate them.