Once I know your PUBLIC key I can add you to my GROUP (adding my and your public key together). Then create ring signature and no one knows who signed message you or me. Then I can fork blockchain !!! (maybe even from genesis block)
Again, you're conflating ring-signed inputs with individually-signed outputs. The trick here is the combination of stealth addresses and ring signatures, not one or the other. I'd suggest you start with the whitepaper and fully grok the maths behind it as a first step towards understanding.