Computation: proof-of-stake is coming - expensive computation won't be a factor. With that said, if you did everything on-chain, that would generate a lot of activity. (In a separate reply, I have spoken of focusing on the chain for governance, and leaving other details in chain-connected registries.)
Proof-of-stake participants are strongly motivated to ensure the integrity and good reputation of the platform.
By analogy, this is a bit like the Netherlands Water boards. They have power and motivation to do whatever it takes to ensure the dikes are in good order. The population are motivated to support them in this. Everyone suffers if the water gets past the dikes: it destroys the platform.
Pure digital goods are easily duplicated with no loss of utility. For example, you can copy a piece of music and lose nothing from that. The value I see relates to scarce goods.
The only area where I can see something like NFTs working is pure digital goods which can only be enjoyed by the keyholder decrypting them. (This sort of DRM also exists in other forms, but the risk of the vendor deactivating the online validation system when they no longer wish to maintain an old game is a real one)