i meant coinjoin, with lots of participants, every block should be mixing transactions by the miner that builds it
I think you mean payjoin? Coinjoin requires a central coordinator which introduces a point of failure when compared to plain old single-wallet transactions. Payjoin is better from this perspective b/c there is no central party but the sender and recipient still have to interact to construct the transaction, which does not work for offline receive. You want your cold storage keys to always be offline. I don't see this ever changing.
Getting miners in on the coinjoin is very interesting. You don't even really need to coinjoin, just charge a premium for a coinbase output. But it makes the miners a target for the state. They are already a target, do we want them to be an even bigger target? This invites a network level attack. I think we need decentralized mining pools first. nostr:npub1lh273a4wpkup00stw8dzqjvvrqrfdrv2v3v4t8pynuezlfe5vjnsnaa9nk is working in this direction with Ocean, his new pool.
Discussion
Miners can't sign with other ppls keys. It sounds like you are advocating for joining the role of mining pool coordinator and coinjoin coordinator. To what benefit?