The earth does not have to be any place on this planet. The original text uses the word “land”, which may be the Holy Land, which again does not have to be any place on this planet.
So, the earth may as well be Heaven in that verse.
Well, we don’t have the “original” text. We have a text, in Greek, that we use because it’s convenient to translators. But it’s just as much a copy as the rest of them.
Christians DO believe in a new Heaven and new Earth and that the old things will pass away. After a final day of reckoning the meek will inherit the Earth, or land. Jesus’ audience at the Sermon on the Mount probably only thought in terms of the land that comprises the world—not a planet.
I should point out that Hell and Heaven are the same place. Sinners spend all of eternity in the company of evangelicals. Can you think of a worse punishment?