The Monitor didn’t explicitly say that Barry Allen as The Flash has to die.
Admittedly, it’s not a perfect theory, as The Monitor did specifically say “The Flash must die,” not “A speedster in a red suit must die” regarding Barry’s death.
It’s difficult to imagine The Flash killing off Barry Allen or The Flash coming to a surprise ending in the same TV season that features the end of Arrow, so couldn’t The Flash substitute Red Death in for Barry Allen as the speedster who dies by the end of the “Crisis” crossover?