With the Full Site Editing project…
With the Full Site Editing project well underway, Ben Dwyer says theme developers need to be thinking about what the future holds for them — and Universal Themes might need to be on that list:
“We that know that the Site Editor is coming. Building themes [with it] will allow users to transition from classic themes to block themes when they are ready without requiring them to switch themes.”
Universal Themes could allow people to create sites using WordPress without the Site Editor enabled. Then, when the Site Editor is more mature, they could begin using it.