Krack WPA2 decrypt only some small pieces of packets
It's depressingly practical. And you can reset the key multiple times, getting the same key stream every time. So if the third or forth time you get guessable plain text content in different parts of the key stream you can combine this until you have everything. 
Your computer does all kinds of communications, it polls stuff, it makes DNS lookups it uses standard headers and standard protocols all of this produce lots of guessable data. On a not busy network we can also use packet sizes to help us find our guessable data. 
In reality finding known plain-text is not difficult.