@wjmaggos You're halfway right.
You say fewer voters in a primary, but your next idea suggests that more people would be voting in primaries because it would be in their interests to have candidates that they prefer.
No, the better argument doesn't go that direction, it simply says that the two-party system is a natural result of FPTP, and we would be better going on to something ranked choice instead of emulating that with the two-party system.
Same conclusion, but the argument that goes the better, more constructive direction.