In the context that there's the Ordinals Chain merge mined with the Bitcoin blockchain, doesn't the miner earn rewards from both chains, and therefore has the choice of which coin to sell to pay for electricity bills/other expenses?
It depends on the side chain construction. If it is like Name Coin, which created its own tokens, out of thin air, then yes, this is just abusing 21 million coins limit, and then you have separate coins. But you can also have something like RSK, where all coins have 1:1 peg with BTC, and where nothing is created out of thin air.
Ok I understand it depends on the side chain construction. But how to create your own tokens out of thin air? How was the upuse of 21 million tokens and what are their benefits?
Does merged mining add extra work for miners
If you mean "can the hashrate increase", then the answer is yes, because those miners are just mining regular Bitcoin blocks.
Edit: If you mean, "does the miner need to choose, which chain to mine", then it is not like that. You simply mine a regular Bitcoin blocks, which are also valid in another network. More than that: you can potentially mine blocks at lower difficulty, which is what P2Pool did (they had blocks every 30 seconds with 20 times lower difficulty, so they had 20 times more blocks, but most of them were present only on P2Pool chain, and other sidechains can use the same trick if they want).
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Yes you are right that mining bitcoin blocks requires something readily available. It is valid through other networks. And it's the only P2Pool that can or has done that.