Regarding investments in Content Sites

So guys, let's stay someone has spent around 10k$ in 6 months in content and backlinks to get around 50k monthly visitors.
Let's say the person stops investing and doesn't produce anymore articles, or backlinks, after reaching the 50k visitors thereshold. What will happen to the monthly volume of visitors in the next months?
Would google still rank you in a perpetual way, gaining around 50k visits in the future?

Or do you need to constantly upload content and invest in order to keep the visitor volume stable and grow it?

Changes don't really happen that fast so it's very likely he's going to keep top positions for quite some time and similar amount of traffic. It may be years or months, depending on many factors and how active are competitors so it's really hard to tell.

Rarely do I see an inactive site stay ranking highly in google

I would suggest you keep building, even if at a slower pace with little content.

A little is 100x better than 0

It depends on how competitive your overall niche is.

It also depends on where you got your backlinks from, as well as the overall quality of the articles.

It also depends on how you interlink the posts that you have published.

There are many different variables at play here.

It's not just a question of investing money into content and backlinks to reach a certain monthly threshold.

Depending on the overall quality of the experience of the people visiting the site, this can lead to the site attracting natural backlinks which can then increase traffic later on.

Having a great content base early on targeting less saturated topics can build up your domain authority sufficiently so that you can pretty much let the site coast.

Of course, this all assumes that your niche target is evergreen.

An evergreen niche involves information that doesn't change all that much on a year to year basis.

On the other hand, if your niche involves a topic that changes rapidly like crypto, you can't afford to coast.

You have to keep investing resources into your online property for it to remain competitive.

Agreed , competition will try to knock you off with better and longer content.
Bottom line keep adding content slowly and steady.
Unlike before google deranks quickly a stagnant blog.

Mostly depends on your competitors and competition of your niche

Keep adding some content to the website. If a website is not ranked regularly, google will punish it.

You should keep adding more content. It will help to rank for new key words and also help to maintain rankings.