From the viewpoint of FIFA, they are not interested in how much it would cost, since most of the money comes from the country that organizes it. They are interested primarily in the possible revenue the world cup would generate. Ethical reasons are also a factor (with the abysmal working conditions of the construction sites in Qatar, but they weren't known at the time of the host selection), so don't expect any world cup in Venezuela or North Korea in the following decades (in the case that they would even want that).
From the viewpoint of the organizing country, the reason they want to host the world cup isn't financial. Almost no world cup (or any major sporting event) will generate a profit, and if they do, it will be relatively small. Big sporting events (like the Olympics, which is notorious for the high cost) always have fewer countries that want to host it. The main reasons are mostly prestige and geo-political influence (in the case of Russia).
Some sources to back up that big sport events don't make a profit:
The reason that they are often building new stadiums is that they don't have enough big stadiums to host an event like this. They do have domestic leagues, but there doesn't exist a domestic league that has enough spectators to justify it.
Since prestige is one of the main targets of the host country, and the construction costs aren't relevant, they mostly opt to build a brand-new fancy stadium, instead of simply renovating their old ones.
TLDR: They destroy old stadiums because they are old or ugly and replace them with newer fancier stadiums.