The rise of mobile devices isn't new. Search behemoth Google announced in May 2015 that mobile accounts for more search than desktop in 10 of its key markets. This is a significant shift, but many companies have not made the shift to responsive websites that are more mobile-friendly. This could be the cause of a subtle but steady erosion of market share for those businesses.
As of April 21, 2015, Google rolled out an update that gives preference to responsive, mobile-friendly websites in searches done from mobile devices. Consider that more than 50% of your potential site visitors are going to access your website on their smartphone. Non-responsive websites will likely appear lower in the SERPs on mobile devices.
The results for non-responsive websites will likely show up as lower overall traffic over time as competing websites shift to responsive design. It may also manifest itself in higher bounce rates for mobile users coming to non-responsive sites, and potentially lower conversion rates for mobile users.
From my perspective, the business case for redeveloping a non-responsive website is starting to be more clear, especially for companies that rely on their websites for new lead generation. It also increases the importance of customer retention and referral marketing. The data that drives the business case is likely sitting untouched in your website visit statistics, so if you haven't done so lately, it's high time to dive into the numbers.
The wind will always blow and water will always flow. Change is constant online and businesses must adjust to survive. Here's hoping you make choices that allow your online presence to thrive in 2016 and beyond.