In the end, it turned out to be extremely difficult to do as you describe. First, static sites, for us, aren't really static - there's dynamic content on a page, there's certain core components that we have to be able to serve, for example, we have to have a mechanism for people to report abusive sites, have to have a mechanism to remove those sites, and we also have to have the ability for users to request the deletion of these sites, and so on.
In the end, it was going to come with a significant amount of engineering cost to continue to deliver static sites, even with reduced fidelity and a worse user experience. We felt it was better to be clear with users that we are going to transition them to the new platform along with providing them with a backup of their content to insure nothing was lost.
I'll say that the vast majority of current classic sites are extremely old and inactive, and not receiving any traffic in the last two years (editors or visitors). In addition, more than 90% of users currently choose new sites when they are building a site, so we are giving users a long window of notice to decide how they want to handle their sites, whether active or inactive.