Not sure why anyone would think once you achieve a good ranking on Google or Bing that now you're all set for eternity. Search engine positioning is a trial and error endeavor. Believe me, success is transitory. If you're on the top today, that's likely going to change tomorrow. Your ranking can often change as much as every hour!
Maintaining top position requires CONSTANT tweaks. And nothing is guaranteed... so if someone tells you they can guarantee your website a top ranking, they simply cannot control the search engines.
BTW, I think that's a good thing! If someone figured out the secret formula to getting and keeping a top ranking, Google would write code to change that.
The game is changing every day. Pay attention... and make your content relevant to your visitors.
And with Squarespace's SEO friendly websites, it makes it really easy to keep your site fresh.