if they blacklist coins, eventually, there would be no coins left because there is a fixed numbers of allowed bitcoins to exist apparently
To take an extreme example, imagine that ALL bitcoins have already been mined, and there are no more bitcoins to be found. Then imagine that they start blacklisting bitcoins at an incredible rate: HALF of all bitcoins in existence are blacklisted each year. Remembering that it is possible to subdivide bitcoins (and arbitrarily far, given minor changes to the protocol), in what year will there be no coins left?
You falsely assume there'd be a fixed rate of coins lost to theft. When there's 100k coins left, though, it's entirely possible that someone goes and steals all of them.