But how long will it going to exist?
Well the fact is, that every blogger can place ads provided by Google and earn some penny's. But Google also earns on the ads that you place on your blog. The blogger keeps only a bit of the profit, the rest of the profit goes to Google. And that's the reason why Google BlogSpot exist. The costs of providing host-space for free is also paid with the income from the ads you place.
Not everyone places ads, and even for them it's going to be no problem at all, because most of the people placed ads. It's simple, if Google continues to earn on the ads they provide the blogger, that places the ads on their blog, so BlogSpot will exist as long as Google makes profit on the ads.