Adding pages to mns ranking on yahoo/bing

One of my old NMS from like 2011 is still making decent money with just 1 very shitty article and ranking high on bing/yahoo. I am a little afraid to change/improve the article as it could potentially loss it's ranking.

I am also not sure if adding any new pages will bring any positive ROI on y/b ?

If it ain't broke... as they say, just let it keep bringing in the money. Set up a new site and try and mimic what you did on the original site re: back-linking and on-site optimization, and then try adding new articles to that. See if you can outrank your original site while targeting more keywords.

If it's just one page site and still bringing good money from yahoo/bing then I will not touch it to be honest, just let it keep bringing the money.