As explained in the previous answer, if you stop doing SEO it won't mean an instance stop in traffic. It will slowly reduce over time however because there are two parts to SEO. On-page SEO and off-page SEO. On-page SEO is about keywords and structure and so that remains on your website for all time until changed. Off-page SEO is about backlinks, and citations and getting your site's authority increased in the eyes of Google. This requires an on-going commitment and the "power" of the off-site SEO will diminish over time if not maintained. It costs you a lot less to stay at number 1 than it does to get there.