Many companies out there are getting to reaching a milestone where they have had their website up and running for 5 years plus now and are happy with the results. When it is pointed out that their site can’t be read very easily on a mobile device their response is usually that the site is meant to be informational rather than functional so not worth the investment to have an overhaul.
However we reached a tipping point last year where more sites are accessed on mobile devices than on PC (52.5%). This has lead Google to issue a warning to web developers that they will be see an increase in visits from their Smartphone Googlebot. If your site is non responsive you will find that your Google rank suffers. Some reports have indicated a 50% drop off in traffic to sites.
This will result in your competitors having an advantage over you in the search results but more importantly you risk visitors abandoning your site prematurely which will result in a loss or leads/sales.