I read that the mutated strain is 50% to 70% more infectious. In simple terms that means for every ten people infected by the original strain, 15 or 17 will catch the new one. In other words, it is a mathematical certainty that the new one will take over

It reminds me of the scene in Titanic where the designer of the ship had its blueprints out and was calcualting the amount of water being taken in by the breaches in the hull and announced it was a mathematical certainty that the ship would sink. (btw, he was one that made it off the ship.)

Because the original strain is so infectious it gave it that much more opportunity to improve its efficiency. Every new human was one more opportunity to experiment in improvement. So the 70% more infectious one gives it 70% opportunities to become even more infectious. The vaccine is the only thing that will reduce the virus's opportunties to improve itself.