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As you know, Google said over 50% of the search results they deliver were processed through the mobile-first indexing mechanism. But Google still has another 50% or so to go and that next 50% will be the harder batch for Google to migrate over.
John Mueller from Google confirmed this last night saying the next 50% will be harder because the ones they did already migrate have more parity between the desktop and mobile versions than the next 50%. For example, this site has issues between the mobile and desktop version (yes, working on a redesign). So the challenge is when a mobile page doesn't match the desktop page in terms of content, links, structured data, etc - that can cause issues.
John said Google plans on communicating via Google Search Console to tell the webmasters what needs to be fixed sooner than later to help the site migrate over to mobile-first indexing. He also said he suspects a number of sites will never be mobile-first or mobile-friendly and thus it is what it is.
So if your site has not yet migrated over, you are part of the more difficult batch of sites/pages to move over to mobile-first indexing - congrats!
Forum discussion at Twitter.
