With Search Engine Optimisation, you have to remember that there are millions of other websites all aiming for top results. Your placement on a search engine is never guaranteed. Everyone is striving to obtain that Page 1 position, however, some companies use "Black Hat" techniques which is a method of "foul play" to improve your page rankings. Not so long ago, BMW used such techniques and were penalised and removed by Google from all listings
Although organic "White Hat" techniques take longer, they perform much better long term and do not have any risk involved. By providing rich content that everyone wants to see, you naturally improve the importance and relevance of your website.
So in answer to the question - You will remain at your top position if no external factors change such as competitiors SEO. However, as you continue to pay us a monthly fee, we will continue to improve SEO on your website to help improve and maintain your position.