An “ordinary user” is someone who installs an application to get its job done. He’s not interested/willing (or capable) to study all the internals of an application or spend days to find out to setup a simple working environment.
Of course repos can be cloned into Packages folder manually without Package Control, but it already requires investigation to find the source repo, destination folder and several manual tasks, which are easy for an expert or power use, but a burden for new users. If you don’t believe that, just skim the forum for all the “why is this so complicated” complaints. Hence it’s not too wrong to call it “literly impossible” from a user’s perspective.
I don’t complain for my self. I guess I’ve learned the least required lessons to work with ST, but the status quo is why I am the only one left in our company. All other’s have moved to VS Code due to its simplicity in setting up new environments or exploring and installing required extensions. It’s even harder to convince new engineers.
Sure, Package Control can be installed with one click, which is easy, but that was not the point of @PetrKrysl’s first comment about this topic. All reactions prove you guys either haven’t read or understood his comment.