This article is omitting a lot of side expenses.

Let's take Paris case: if you use Ryanair as stipulated, you'll end up in Beauvais which is about 80kms from Paris. You'll need to add a 20€ mandatory bus fare or find a car sharing in order to arrive in the city.

The idea is cool and definitely doable if you are highly organized. But you'll need way more preparation than this vague travel roadmap...