The term "legs" has been around well before video games. It was mostly tied to movies grosses. If a film made a lot of money compared to its opening weekend take, it would be deemed to have legs. For instance, Titanic opened to around $28.6 million but ended up making $600 million domestically. Those are huge legs. Not only because it continued to make money for a very long period of time, but also because it made a great deal of cash compared to opening weekend. Over 21 times the opening weekend take.
Changing the term to fit your own private definition is really silly. De Blob most certainly has legs. It started off very slowly, but gained momentum. Putting some disclaimer that the game needs to sell a certain amount per week goes against the idea of "legs". It's not how many games sell every week, but if it stays at a constant pace that determines legs. The biggest indicator of that is the comparison of opening week to overall sales.
With your new changed definition, only blockbuster games can achieve legs.
