What is the alternative?
Any service at some point when it gathers enough users can turn into a scam. What is the suggestion to avoid this scenario?
we assume that p2p cannot be a solution for all cases.
The alternative is coinjoining. Coinjoin transactions are strictly superior to "mixing sites" since you can't get scammed out of your funds, and no third parties are trusted not to leak your privacy. The forum admin strongly recommends coinjoining over "mixing sites":
Congratulations to the Wasabi and JoinMarket developers! JoinMarket pioneered a lot of CoinJoin science (and BTW, belcher wrote an excellent & comprehensive wiki article on privacy), while Wasabi is the first wallet that implements CoinJoin in both a highly-usable and sound way. As both a signer and a donor to the CoinJoin bounty fund, I'm thrilled that these two pieces of software exist!
For everyone looking to improve their privacy, I highly recommend checking out Wasabi, especially over centralized "mixers".