KYC itself won't and would never deter bad player's, because these are willing to go the extra mile just to gain access to such platforms. AFAIK KYC is 50/50.
More like 5/95.
Because you can buy a fake ID to evade verification.You can pay a homeless person or a drug addict $1000 for their ID and abuse that.
[Off-topic, but that is probably how the phishing scammers verified themselves on domain registrars. I highly doubt he (it's a one-man show apparently) would use his own ID for all those phishing sites.]
KYC by itself is NOT effective unless you implement it like the big centralized CEX do and use other heuristics to suspend the account or trade.