Sure it is. Both Apple and Google, through various tactics, have ensured that it's virtually impossible for a third smartphone OS to be successful to anywhere near the level they have been.

Android is fine. It has some downsides vs. iOS, and some advantages. But that isn't the point. The point is that to make a new smartphone OS (or even one that's based on Android, but is independent of the Google ecosystem) that can do everything Android and iOS can do is an undertaking that few would even bother to take on. That's not due to technical challenges, it's due to market barriers that Apple and Google have erected. (IMO, the sorts of things that we as citizens in a healthy society should not allow corporations to do.)

And those that (sorta?) do try to make a competing OS, like LineageOS, GrapheneOS, CalyxOS, etc., end up with far less-capable phones than a Google-blessed Android phone. (And when most/all of those capabilities are present, it's through brittle hacks and compromises that basically turn the phone into an imitation of the Google-blessed phone, with many of the downsides intact.)

Put another way, it's not Apple's or Google's responsibility to make things more competitive, but it is their responsibility to not make things anti competitive, and it is their fault when alternatives don't exist because of their anti-competitive behavior.