Stated by both Ramsey Russo and the Speed Forces during the Last Temptation of Barry Allen. Then E-90 Flash won’t replace him and he will died along with the remaining Earth 1 Heroes from the anti-matter wave.
Then the Speed Force said that Ramsey would just use Barry like a mindless puppet. Cause he still gets destroyed in Crisis. It's just his knowledge and speed that stays in Ramsey's collective life blood.