Studies done in later years show that those limits cater for people totally unused to reading subs and in reality reading speed for most people is much higher than that. There is a thread here on this, you might want to have a look after this. How quickly can a viewer read subtitles?
And especially Viki viewers are even more used to subtitles, because Viki specializes in Asian drama and you don’t find Asian dramas dubbed in English (or if you do, it’s a very rare and recent phenomenon).
As a professional translator of more than 4 decades, I can tell you one basic rule: adapt to your intended audience. In the same way as a scientist won’t write in the same way for a scientific journal and for a divulgative magazine article intended for the general public (e.g. “Focus” or “Time” or “Nature”).
There is a very specialized sort of audience here on Viki - and newcomers very quickly adapt. For instance the use of Korean (or Chinese or Japanese) appellatives instead of just using people’s names. We wouldn’t do that in subs created for other platforms, but here it is the norm, we feel that they give information on hierarchy and relationships. Also some expressions, interjections, food names etc. left untranslated, because we all know what they mean.
And yes, many people watch shows on a phone. Especially on long commutes to work or school - people who first are bored and secondly are too eager to know what happened last night on their favourite show. I used to scoff too, because my eyesight is not that great and a phone screen is too small. This until I had to spend months in a hospital. My phone was my saviour then (I have unlimited data, he he).
Another thread that you might be interested in reading is this one. I’m pointing it out since you are new here and it’s an old one, so you wouldn’t find it easily: