Once people realize that when they transact a BTC transaction and all their history can be tracked, they won't like it. Most people think they are hidden behind pseudo-address, but that really is not the case anymore. Go figure. Imagine your bank account is transparent. Every transaction can be seen by everyone. Scary stuff... And it's not about illegality. The fact that everyone can know how much money my neighbor has or his spending habits whatever. Just horrible horrible horrible. It's awful. I can't wrap my head around it. And it's only gonna get worse with more KYC. Coinjoin isn't practical. There is nothing that can fix this. Some people already find this troublesome. You might not feel bothered holding 0.1 BTC, but if you have more than that? People are crazy. It maybe isn't problem now, but the future?
Financial transparency won't prevent crimes. And KYC doesn't prevent crimes either.
Optional privacy also isn't a solution. All transactions must look the same or very similar otherwise you stand out.
Monero solves all of this. It still has a long way to go though.