Your point that "only criminals would use throwaway phones to register accounts" is completely wrong (I'm surprised you haven't even used "terrorists" witch such a line of reasoning). There's no site that I want to give my phone to, and I'm certainly not involved in any illegal activities. Ever heard for privacy in the digital age? Anything you won't protect will be sold and replicated in as many databases as possible.

If some site like Google would insist on the phone number, I'd buy a SIM card only for that purpose.

But now there is a push to even not accept such "for one purpose" phone uses. Bad, awful for privacy.