Unix is not really a desktop system. It's a system running on a large computer which costs about as much as a house located somewhere in your university's basement. You, as someone who cannot afford his own computer, have to share the computer with two thousand others, and with several dozen users simultaneously for that matter.
Incidentially, you can nowadays also run a Unix-like system on your desktop computer or on a credit-card sized SoC which costs $20.
In principle, however, Unix isn't designed for single users. The single user isn't important. What's in your home directory is your problem, but what
root can do is everybody's problem. Therefore, only the few tasks that really require you working as
root should be done with that user, and preferably (to limit the time window during which you can do harm) not by logging in with that account, but by explicitly using
sudo for the single commands that require it. There is a lot of religion in that as well, which is why some distributors are so darn arrogant as to threaten you when you type
su rather than
sudo for every single of the 10 different
apt commands you have to run to install some petty thing.
So you can erase all your personal photos without being
root. That's right. Malware can erase all the stuff in your home directory, that's right. It can deny service by filling your disk until your user quota is reached, that is right. But from the system's point of view, that's just your problem, and nobody else cares. No other user is (in principle) affected.
Now, the issue with a modern single (or few) user system is that the bivalent logic security model is quite inapplicable, just like the "there's hundreds of users" idea.
Unluckily, it is very hard to come up with something better. Look at Windows if you want to see how to not steal an idea (they really managed to make a bad approach even worse).
Some web browsers and phone (or smart TV) operating systems attempt (and fail) at providing something better, and modern Linux has a more fine-grained system, too (but I wouldn't know how to properly set it up without spending weeks of my time).
The problem is that the bivalent security model assumes that normal applications do not require any privileges (which is wrong because some mostly-harmless things do require privileges) whereas non-normal applications require full access to the computer system (which is also wrong, almost no program needs full access, ever).
On the other hand, even finer-grained security models (which still are pretty coarse) make the wrong assumption that if an application requests a set of privileges, it really needs that complete set and the user is comfortable with granting it.
There is, to my knowledge, no system where an application can request the privileges A, B, and C, and the user can agree to granting A (but not B and C), and the application can then query what privileges it was given and decide whether it's able to perform the requested task or not.
Thus, you generally have the choice of granting XYZ-app "store data on permanent store" (which you're maybe OK with) and also allowing "access my location" and "access my personal data" or "install system driver" (which you're not OK with), or well, you can not run the program.
Or, you can allow XYZ-program to "make changes to your computer", whatever that means, or you can choose not to run it. And, you have to confirm it again every single time. Which, be honest, really sucks from a user perspective.