I don't really disagree with this. But it is all about tradeoffs. There is no magic bullet.
The vertical approach of Amiga, Atari ST, Acorn, Apple and others may not have given the same variety of choice in periphrials but they have other advantages.
All of they demonstrated technically superior products in almost every area compared to the PC. And that was the case for quite a number of years despite a much higher sales volume of PCs.
In a world where that sales volume was taken by these competitors, things could have progressed very differently.
I also don't think it is given that all of them would have persisted long term in using propritariy solutions for everything. I think they also would have seen the advantage in having more common open standards.
There is a flip side to what you talk about. When Apple made clones exactly the same happened as on the PC. Prices dropped but you got a race to the bottom, where the quality of the Mac as a platform fell.
You quickly get into a prisoners dilemma type of situation, where you end up pursuing local optimization over global optimization.
You see this pattern in almost any market. Competition on equal terms is often a powerful driver to get prices down. However a big problem in such a setup is that it drives every particiapent to rally only look slighly beyound their own nosetip. It is all about whatever tiny little incremental improvement you can make to gain an advantage.
It is like software development. You end up following a strategy that piles on technical debt as you never address larger scale problems.