Nice post and interesting topic
I have done countless KYC with FIAT bookies and of course never had any problems with being accused of multi-accounting (I was harassed with other issues, but that is not the topic here). So indeed the industry standard does work and even protects yourself against the bookmaker - but both only to a certain extent.
Looking from a (FIAT) bookmakers perspective, KYC isn't the holy grail though. The reason to do multi-accounting is mostly for new customers bonuses or some other kind of free money. And because there is quite a lot of free money to get, and people would do a lot to get this money, even bookmakers with strict KYC have to take additional steps to keep multi-accounting at bay.
If you take a look at the T&C`s for some first deposit bonus, you will see a lot of restrictions. Certain countries with a history of multi-accounting are excluded, deposits with Skrill/Neteller are excluded, the rollover requirements have been upped. Bookmakers had to put these additional conditions in place, because even with KYC, some users used very sophisticated methods to still register multiple accounts and cost them a lot of money. And even with all this, there is still abuse happening.
Now for crypto bookies, I am not sure, if this would work for them. Technically it would work of course, but economically I am not sure. As you mentioned it's their USP and you would have to figure out, why people are playing at crypto books/casinos in first place. Privacy reasons ? The odds ? Not being able to use FIAT to deposit/withdraw ?
By adopting KYC, they would probably lose a lot, if not all, of customers that value the privacy aspect of things. You might also lose a good amount of people that can't use FIAT - playing at crypto books/casinos without KYC is really easy and convenient. Registering takes a minute, depositing is easy and fast, withdrawals are fast-ish. By implementing KYC, you are building a hurdle, that a lot of these people might not be willing to jump over. Recreational players just want to gamble a bit, without too much paperwork and hassle.
For me personally, I am fine with doing KYC at more or less all FIAT bookies, but very hesitant to do with crypto bookies. Crypto bookies always have some shadiness surrounding them imo. You never know who are the people behind it, they have some weird licenses (if any) and are more likely to vanish on a daily basis or scam you. I just don't trust them that much like I do with FIAT bookies, especially with the well established ones. When I speak of trust, I am referring to A) my money being safe and B) them not selling my data.
So in the end, I think the crypto bookmakers just have to live with multi-accounting and we as betting/gambling people have to be cautious playing there, since you are always in danger of being accused of multi-accounting for the reasons you mentioned. The gambling community just has no lobby and we are always sitting at the short-end of things. Just adapt.