All right then. The metacrisis was a three-way exchange of regenerative energy, resulting in two Time Lord humans and one Time Lord, the entire process sparked by the fact that it was going to happen and always would happen, and the echoes it sent through time. If it hadn't happened, there would have been no echoes, hence no metacrisis, hence a definite ten regenerations, hence a very likely collapse of reality itself, and then nevermind Acumen or Sentience or Marina, because nothing would exist.