How do you measure success in SEO? Long ago, it used to be that you could take a quick scan of Google and see where your pages ranked and simply make a judgment based on that. But we are now living in an era when you can no longer look at just one number, or set of numbers, and make snap judgments. From a holistic perspective, mere ranking doesn't really tell you much.
Instead, you should be asking: what is it that you are trying to get visitors to your site to do? Is it merely to receive information? Do you want them to sign up for a milling list? Or to get information on a product, or to place an order? From that perspective, it is highly worth your while to examine how successful your SEO strategy is through that lens: which search terms are leading people to which pages, to take which actions?
Google Analytics can help you with this research and can generate numbers to back up specific statements about SEO results. It will allow you to look at organic and nonorganic traffic, compare search term results over time, and filter out branding keywords to see how much of your traffic is coming from generic terms. But it's also worthwhile to look beyond Google, to see how much of your traffic is coming from the the major search players (Bing, Yahoo, and possibly foreign search engines), and to divide that traffic by the current relative popularity of each search engine.
Take a look at the same data over time and see what the trends are; are you seeing a seasonality to your traffic? Are there certain keywords that have become more popular over time?
If you can identify which keywords are leading to the most profitable actions on your site, you can narrow down your improvement efforts to key pages and actions; those are the ones you should address first and foremost, before engaging in a complete site overhaul. SEO is a potentially limitless science; there are endless permutations and factor combinations of data you can produce statistics for and try to argue certain points with. But by taking a "bottom-line" approach, as opposed to merely an "attention-receiving" approach, you can justify SEO from a cost standpoint, and make a case about how worthwhile it is, and has been.