Furthermore, if we lived in a world where the two main OS's were locked down to an insane degree, we would also have plenty of alternative operating systems. The reason we don't today is because we don't really have a need for it, in the same way linux has a monopoly on servers and nobody really cares.
Those make up 0% of the market [1], which classifies Apple and Google as gatekeepers.
[1] https://gs.statcounter.com/os-market-share/mobile/europe/
If you have a market with a handful of companies producing good products, and a handful of companies producing shit products nobody wants or buys, you cannot claim that the companies producing the good products are "gatekeeping", and that's the reason why nobody buys the shit products.
It doesn't matter how they became gatekeepers.