The problem is that leaves you with an unstable situation under FPTP. Let’s say that our fictional third party, the Yellow Party, is to the point where 40% goes to Republicans (right), 30% goes to Democrats (center-left), and 30% goes to Yellows (left). Now Republicans are winning despite Democrats and Yellows forming a majority. So Democrats are going to split at some point, arriving back at an equilibrium of approximately a 50-50 split between Republican-Democrats and Democrat-Yellows. So in essence, you’re right back where you started.
But you have a new party with better politics. Otherwise you argue that Democrats and Republicans already represent exactly the will of the population.
The problem is that leaves you with an unstable situation under FPTP. Let’s say that our fictional third party, the Yellow Party, is to the point where 40% goes to Republicans (right), 30% goes to Democrats (center-left), and 30% goes to Yellows (left). Now Republicans are winning despite Democrats and Yellows forming a majority. So Democrats are going to split at some point, arriving back at an equilibrium of approximately a 50-50 split between Republican-Democrats and Democrat-Yellows. So in essence, you’re right back where you started.
But you have a new party with better politics. Otherwise you argue that Democrats and Republicans already represent exactly the will of the population.