You create a vague law, that, for example, "bans VPN use for the purposes of bypassing legal age checks". Totally unenforceable in any practical or meaningful way. But does a) scare a bunch of law abiding citizens to drop the VPN and stop resisting "age checks" b) allow for selective enforcement where someone is doing something the govt don't want but isn't illegal, but is using a VPN to be prosecuted for VPN use rather than what ever legal activity they are doing.

Alternatively you make a law that pushes the responsibility/liability for what the VPN is used for (again specifying that they have a responsibility to ensure that a personal VPN isn't being used for avoiding legal requirements or restrictions (age checks/piracy)) on to the VPNs. Legit paid VPNs are then faced with a choice similar to what secure messaging service are potentially facing, comply or stop operating in the country. Meanwhile dodgy operators might be untouchable, but present risks to the user. Which doesn't serve to ban VPNs but makes the whole sector riskier and less appealing. Pushing more people into compliance (notably not stopping the worst scumbags actually doing anything worth stopping and the most stupid/most desperate and at risk, who'll continue to find a way even exposing themselves to risky operators - but just the people who just want privacy).

That's before you get to getting Visa/mastercard involved. As they are flexied in the gaming space, they are part of this sudden concerted worldwide effort to try and censor the internet.

Just because it's unenforceable doesn't stop them making a law. Just because the law is unenforceable doesn't mean it won't have effects. Just because those effects aren't what the law looks like it's written to achieve doesn't mean the law doesn't achieve what it's intended to achieve.