What Will Happen if You Don't Update?
In truth, until Google actually launches its newest update nobody really knows, although there have been a few “give-away clues”. One of which was back in February when Google launched its useful “mobile-friendly test” that shows just how Googlebots perceive existing websites and whether they are mobile-friendly or not.
In short, the new update is set to improve rankings of mobile-friendly sites that enhance user experience on mobile devices. Websites that are not run the risk of being demoted. However, it's important to bear in mind Google's mobile-friendly update is set to affect mobile search results only and not those done on a laptop or desktop computer.
On top of this, the algorithm is set to be applied globally and will be done “page by page” in real time. What does this all mean? That every page will be judged separately to assess whether it is mobile-friendly or not. In short, if some of your pages are not but others are, you might just find your site would not be affected or adversely ranked as being unfriendly depending on how many pages are friendly and how many are not.
Google's End Goal
Google's end goal is to enhance searcher experience with their new update turning frustration into a more positive and seamless experience for mobile users. If you are among those who have failed to get your website mobile-friendly, then it might well prove to be a rather expensive oversight resulting in a loss of revenue due to the fact you'll be missing out on mobile searches to your site.
Resolving the Problem
Fixing the problem may not prove to be a massive endeavour and could be as easy as getting a developer to change a few simple things, namely to remove blocks on certain files and to update the robots.txt file, both of which are things a developer would understand and fix in a jiffy. At the other end of the scale, the task might be rather more in-depth and could mean a complete site redesign so that it's more responsive incorporating the all-important mobile website “best practices”.
In a worst-case scenario if you have not updated your site to be mobile-friendly, you may find that all your sales-driven organic traffic that comes from mobile devices simply vanishes altogether when Google's update comes into force With this said, it's thought the worst that could happen is for results to show a decrease of about 50 to 80% which is still something that should get you worried enough to make sure your site is mobile-friendly right now rather than risk waiting any longer to do so.