Even though the benefits of coinjoins are well documented and clearly help us obtain great privacy, conjoins introduce separate transactions into our usual spending flow.
You may be aware, but other types of transactions exist today to also take back some control of privacy. For instance, people can use Payjoins when supported by their wallets.
Payjoins via BIP-78 look like regular transactions and involve both the sender and receiver contributing inputs when transacting. This could be the merchant themselves or potentially even yourself (via 2nd, 3rd, 4th wallets).
Even though its early and that they are not so commonplace today, Payjoins are potentially an even better solution or a natural evolution from coinjoins over the long-run. Especially when in future they support more than 2 participants.
Payjoins make it near-impossible to know the true amount paid and they are indistinguishable from other transactions. There is no obvious change or obvious payment output. And of course you can use multiple inputs. There are so many different potential interpretations of the likely amounts, that the bread-trail and 'heuristics' used by surveillance companies can come to a dead-end.
With the release of the PDF (Payjoin Dev Kit), we should now see business adoption grow, since it makes their own transactions more efficient and more cheap. Outputs for the receiver (the merchant) will allow transactions and outputs batched together to save on fees in future. The privacy argument for merchants may almost be a secondary afterthought.
@bitgould for instance has referred to this as 'transaction cut-through'. Kudos to you Dan, on dedicating so much time to this Payjoin initiative!
During a significant bull-market, exchanges will also be incentivised to use Payjoins. Firstly, exchanges could benefit from batching transactions. However exchanges may also struggle for liquid UTXOs to have ready to fund withdrawals at times, even if they hold 100% reserves - due to a cold and hot storage setup. With Payjoins, exchanges would have the ability to directly fund the withdrawals with the incoming UTXOs from their other customers. Again, skipping entire transactions and saving a bunch of fees.
Further benefits of Payjoins are discussed here on Github.
Even if Payjoins only account for 10% of transactions on-chain in the coming years. It's enough to reduce the "common-ownership heuristic" to a 90% probability, such that multiple Payjoins in sequence could reduce that from 90% to 81%, then 72%, 65%, 59%, 53% etc.
Of course with the incentives in place, it is hoped Payjoins will soon make up a greater share than 10% of transactions. Taproot transactions already account for 60% of the network, despite being less than 2% at the start of 2023.
As we stand today, Payjoins aren't so commonplace, but they have hugely significant implications for privacy onchain. Hopefully we see the momentum shift over the coming months, with wallets, businesses & individuals adopting Payjoins.
You can also save on sweeping dust and joining all of your kyc coins from different sources to one (or multiple) addresses, so there's uses beyond privacy. Take all of your coins from electrum, ledger, strike, etc... all to a shiny new Jade in one transaction :-)