> the lines have to clear.
Sorry, the lines have to clear what? Surely there must be some kind of constraint on "lines" that they have to overcome.
I mean that at the bottom of the Tetris board, the lines need to vanish so that the Tetris board keeps moving downward and doesn't grow unbounded.
The lines (rows) in Tetris have to become complete and then disappear to make room for the new falling pieces.
In code the thing has to become stable, can't just keep packing more and more noise onto it.