Just like with many things in life, just because you can doesn't mean you should. As an example, I can technically go and purchase a new car today after getting a loan. But whether I should is another question...
If you upload evidence, what confirmation do you get to prove you uploaded it?
If later USCIS server lags and erases your upload, how can you prove you replied?
If IO decides not to check online uploads since instructions tell you to reply by mail, what whould happen?
With mail you can have proof of sending and proof of delivery.
Good luck!