The idea that, for many people purchasing an iPhone, the inability to install apps from alternative stores or the web is a selling point seems rather questionable.
On top of that this could easily be provided by an optional switch that enables you to enforce this on your own device if you so desire.
It is a poor reason to request that all app distribution for everyone else also has to go through Apple and its terms, basically for your convenience.
And that still plagues every computing platform now. Including Apple platforms. So the lockdown clearly didn’t completely solve that problem like a lot of people seem to claim