backups

You do not have enough money to hire someone to watch and protect your website.

If you do not have enough money for the basic protection of your revenue generator (or do not possess the technical skills to do it yourself), then I don't think you are in a position to quit your day job to rely on that revenue generator.

Besides backups, you should have a contingency plan to put into place in the event your server is down for any reason (hacking, power/network failure at hosting company, earthquake or other natural or man-made disaster). Your level of dependency on that revenue source should determine the amount of money/time/effort you put into preparing against such down time.

There are some companies who could survive without their servers being online for a few hours or even a day or two. A contingency plan is not important for them. Other sites can lose thousands of dollars for every hour they're down. They are more likely to have a completely mirrored hardware setup (with the most recent backups) at another physical location, ready to go live at a moment's notice.

If you have a valuable site, you need to take steps in proportion to its value to you. And simple backups will not be sufficient for a valuable site.
In the last two months I have read of two very valuable sites that were the subject of serious malicious attack - WebHostingTalk (posts about it here WebmasterWorld), and now yesterday we read Avsim has been completely wiped out and will not be back with any of their valuable content - see below.

Flight simulator site Avsim has been "destroyed" by malicious hackers.
The site, which launched in 1996, covered all aspects of flight simulation, although its main focus was on Microsoft's Flight Simulator.
The attack took down the site's two servers and the owners had not established an external backup system.
The site's founder, Tom Allensworth, said that the site would be down for the foreseeable future and was unsure if would ever go back up.
"The method of the hack makes recovery difficult, if not impossible, to recover from," Mr Allensworth said in a statement.

Say, you have successfully created a very popular website all by yourself and the Google Adsense money is now starting to roll in

Money doesn't roll up hill, you're going to have to kick and claw every dime up the hill into your accounts, it's not as easy as some MMO sites claim.

You are smiling now and you quit your job just like Markus did.

We can't all be like Marcus, that's why we talk about him. There are those who make much more than Marcus however and they remain quiet, their sites do the talking without a personality pushing traffic. For every such site though there are likely 500,000 or more sites that won't allow you to quit your day job with. (100,000 new blogs are created daily?!)

As for protection, offsite backups are important, even if the offsite is your computer and the backups are manual because that's all you can afford. Don't forget to back up the databases, they're likely more important than the websites files!