You seem to be forgetting, we had your “at least a half-dozen competing companies" situation in the past. It was called the late 2000s and the early 2010s. And the reason it disappeared is not the traditional “everybody merges until only two or three are left standing”, it’s because iOS and Android were so much better than the competition that every other phone manufacturer starved to death or switched to Android.
By the time a new generation of smartphones with sufficiently-equivalent OSes had arrived — in the form of webOS and Microsoft Phone — iOS and Android were so established the newcomers couldn’t successfully compete.
Even today, while there are only two major OSes, there are still numerous successfully competing manufacturers: Samsung, Google, Apple, OnePlus, BLU, Lenovo/Motorola, Huwaei, Xiaomi, Vivo, Oppo, etc. They all make mobile phones, with varying levels of market share across the world.
