Again, assets on a blockchain only matter if someone cares about that blockchain. If I only support blockchain B, it doesn't matter how many assets you have on blockchain A as I will never support it. If I'm invested in blockchain B there's little reason to add to my expenses to support other blockchains.

Even in the most practical terms, if I sell in game assets they're ones I made for my game. I know they work with my game. Why would I put in extra effort to import third party assets I don't make money on? If your assets are in a proprietary format I'd need to spend more money licensing some importer or build my own.