Suggesting that regulating ISPs or SIM cards or even public WiFi is an alternative to the OSA or age and identity verification is ignoring the reality that mandatory filtering of internet connections has been a legal requirement for a very long time now, and it has been 'voluntary' (by the ISPs themselves, opt-out by customers, pushed on by Cameron) for even longer.
It is not a new or novel concept. There are legal adults taking part in these conversations that are simply too young to have ever experienced internet connections that weren't restricted and filtered mandated by legislation, and they would have been teenagers that were old enough to have a say in the conversation when the Conservatives were debating the OSA in parliament.
Mobile internet connections have been filtered since 2004 even, so it's entirely likely that this would also be true for some people that are pushing 30 today. The debate on whether it's appropriate for internet filters to block access to Childline, the NSPCC, the Police, the BBC, Parliament, etc, is 15 years old at this point. Fifteen.
The false dichotomy that exists between the entirely authoritarian measures of the OSA and the still fairly authoritarian measures of mandatory filtering serves only the interests of borderline monopolistic American tech companies who are in a position to weather such regulations as they stifle and snuff out any possibility of a less harmful web ecosystem, and people will cheer it on as they believe the social media platforms they blame for causing harm will themselves be harmed by the very laws they are writing.
The real alternative is not having mandatory filtering but instead voluntary filtering by the parents themselves, which is what everybody seems to think they are arguing for, and that conversation is long since dead. It is entirely beside the point, but contrast it with alcohol laws. The UK is one of the few countries in Europe that has consumption laws both in private(+) and in public, whereas half of Europe only has consumption laws in public while the other half has no consumption laws in either private or public. America on the other hand has many states that prohibit under-21s from drinking alcohol even in private. A better comparison may be content ratings, which are largely entirely voluntary and not a legal requirement.
(+) It's 5+ so there may as well be no laws on private consumption.