Not being blacklisted as spam is a huge market based incentive to add unsub links. A major reason for the mass adoption is that most companies use email servic

Not being blacklisted as spam is a huge market based incentive to add unsub links. A major reason for the mass adoption is that most companies use email services because running your own marketing email servers are extremely difficult. And those companies don't allow you to send emails without one to protect their own email servers in addition to following the various laws in different countries. It's easier to get compliance via these larger companies, particularly when it naturally aligns with market incentives. Regulating a million niche SaaS sites each with an individual custom payments page may be quite a bit harder. But maybe stuff like Stripe will make it easier as a proxy for this regulation.