A while back I launched Bitcoin Armory for the first time only to discover it had to download the entire blockchain(!) before it could seemingly do anything. Does a game like this bloat the blockchain, and if it does, do people care?
Given sidechains are already fairly complicated to understand[1], I'm not sure I get why suggesting their use without showing the use-case here makes a lot of sense for building Blockstream's business model. I guess that's the difference between promoting a company and contributing in a conversation.
[1]: https://www.elementsproject.org/elements/asset-issuance/
Then only deposit or withdrawl of coins will "spam" the blockchain.
It's also worth mentioning a sidechain or payment channels can be "flushed" to the Bitcoin Blockchain (syncing up the balances of off-chain debits and credits) and this is not a trivial operation. If there is a community effort to a robust solution to this problem that other virtual worlds can also implement and inter-operate, makes total sense for us to test it and support it.
Blockchains are cool because they solve the problem of a global ledger without centralised trust. If the problem you're solving does not require the lack of centralised trust, it's just a really complicated and resource-heavy way to implement a balance database.