>It could also go the other way, right? Opening the door to a more onerous version?

I don't see a plausible scenario where the implementation of this mandate makes further mandates more easy to get passed. An age field and an API to access it is as trivial as it gets. More onerous age checking is not something that is an extension to or somehow made more easy given the pre-existence of the age field. No argument against more onerous checking is undermined or rendered less severe due to an age field already existing. There is no slippery slope here.

>So, assuming parental controls were correctly implemented, why do you think this is "least bad" including parental controls?

There is already a pretty significant market for parental controls, so presumably if their quality were a limiting factor in their adoption the market would have responded already. Parents simply aren't interested enough or savvy enough to apply them. Parental controls also just intrinsically suck for a lot of reasons. They are either mostly ineffective or wildly intrusive, like giving total access to children's communications and internet activity to external companies.

>Thirdly, this "age verification" doesn't actually verify anything, because underage people can just choose "adult" anyway. What do you have to say to that?

Presumably an adult is involved in purchasing devices and setting up accounts for their young children. Putting an age of account holder field into the account set up workflow seems pretty effective. It's not 100%, but it doesn't need to be for it to be a major improvement over the status quo. The lack of verification is a feature of this mandate, not a bug.

>we have a concept of "free speech" which includes the idea that people cannot be "compelled" to certain speech. If people were required to add a "sign here to confirm you're an adult" in every romance novel, that would be fine right?

As those pushing this kind of legislation are fond of pointing out, we have age checks for buying alcohol or purchasing adult magazines in shops. Presumably these don't run afoul of the first amendment. This idea that we can't or shouldn't mandate age checking in some form to access content deemed inappropriate to children is just a losing argument. Again, the writing is on the wall here.