As far as I'm aware the advice against running root all the time falls in two categories:\

1. Security: If I get compromised the attacker will do less damage if they're not able to gain root privileges

Will they? Even without root they can: Encrypt my data and demanding ransom;

Yes, your data, but not everyone's data. You mention this is a single-user machine, but if your user is separate from "root" then there still are multiple users. Similarly, they don't get access to other paths outside of your home.

2. Accidental mistakes such as issuing rm -rf against a system critical directory

Once again: Without root I may not be able to accidentally remove or corrupt /etc or /bin or /usr, but I'm still able to accidentally remove or corrupt my data in /home. The latter is arguably worse than the former

No, it the latter is inarguably better than the former. In the latter case you can only destroy your stuff. In the former case you can destroy your stuff plus everything else. Therefore it is clearly better to not run as root because you can clearly destroy less stuff (your stuff only versus your stuff plus everything else).

So, with root or without root, I still have to pay utmost care when I issue dangerous commands such as rm.

Yes, you still have to be careful regardless--but just because you have to be careful in both instances does not mean that both instances are the same. For example, I have to be careful when I handle eggs and I have to be careful when I handle plutonium. This does not mean that eggs are the same thing as plutonium.

As far as I'm aware the advice against running root all the time falls in two categories:\

1. Security: If I get compromised the attacker will do less damage if they're not able to gain root privileges

Will they? Even without root they can: Encrypt my data and demanding ransom;

Yes, your data, but not everyone's data. You mention this is a single-user machine, but if your user is separate from "root" then there still are multiple users. Similarly, they don't get access to other paths outside of your home.

2. Accidental mistakes such as issuing rm -rf against a system critical directory

Once again: Without root I may not be able to accidentally remove or corrupt /etc or /bin or /usr, but I'm still able to accidentally remove or corrupt my data in /home. The latter is arguably worse than the former

No, the latter is inarguably better than the former. In the latter case you can only destroy your stuff. In the former case you can destroy your stuff plus everything else. Therefore it is clearly better to not run as root because you can clearly destroy less stuff (your stuff only versus your stuff plus everything else).

So, with root or without root, I still have to pay utmost care when I issue dangerous commands such as rm.

Yes, you still have to be careful regardless--but just because you have to be careful in both instances does not mean that both instances are the same. For example, I have to be careful when I handle eggs and I have to be careful when I handle plutonium. This does not mean that eggs are the same thing as plutonium.