We first announced this gradual plan in 2017, with an update in 2019, and now we're providing the detailed plan. The goal was to provide adequate notice for people to figure out whether migration, export, or deletion of their classic sites makes the most sense.
EDIT: to the multiple people that are highlighting that 12 years is not a long time, I should clarify. I agree that 12 years in Internet time is not a long time, but it is a very long time in "how websites work" time.
One of the big challenges in the website builder space, something that Google. Wix, Squarespace, have all dealt with - is that when a user publishes a website, there's an expectation around the fidelity and look-and-feel of that website. The data model and architecture for how you represent a rendered page, how you render embedded content, all are very dependent on how browsers work at that time, and the maintenance of those things can become very challenging.
There was a big shift about ~7 years ago, moving away from a "pixel perfect" layout/raw html approach (Wix, Old wordpress, SQSP 5, Classic Sites), to one where there's some sort of grid-and-box approach where the render is responsive, there's reflow, there's padding and spacing and styling relative to the browser experience. It separates out content from layout to a certain degree in a way that is much more future-proofed and modern. This is how Wix ADI, Squarespace 6+, Wordpress Gutenberg, and new Sites work.
For all of those transitions, there was a platform shift, that required a complete rework of how the system works. My point about "12 years" was that the model of how classic Sites works is 12 years old, and new Sites was intended to match the modern way of how websites are built and delivered.