You achieve high positions in the search engine after work on SEO. When you stop this work, your competitors continue to do it and they will take your place. Search engines change ranking factors constantly. And if by the current factors you are the first, after the change you can fall sharply. You need to analyze the changes in positions in your niche. Seo should always work.
SEO is a race. When you stop running, your competotors will overtake you and you will lose your ranking. If your competitors are also running or walking, you can't stop. I work for a web design company in Madurai and the industry is very competitive here. So, we regularly do SEO for our website.
We will not stop our SEO efforts even if reach the first position for all keywords. The reason is as soon we stop doing it, our competotors behind us will overtake us. Sometimes, it can be very difficult to get back the top position once we lose it.
However, we have stopped doing SEO for one of clients after reaching the top position. The reaon was the industry of that client was not that much competitive so it was unlikely that the competitors overtake our client.
You don't have to do SEO if you have valuable content. Bad content requires consistent SEO, while great content markets it self.
When I say great content I mean great user experience. Was the user satisfied with the page he or she visited? Did they get what they want? Are they coming back? These are all things you should consider.
And no, you don't need to do SEO continuasely.
I heard a great quote from Income School channel on YouTube ...
If you are gonna put hours of work into SEO. You would be better off optimizing and improving your content.
You know, links aren't everything to Google. They look into user behavior too.