Fundamentally, this won't even be an issue. Premades will generally check in with eachother if they are having lag or disconnect issues, although this isn't my main point.
If something unbeknownst to them comes up and one disconnects, then it will still count as a loss. So what? Big deal. 4v5's were an automatic loss before so I don't see a big change in that aspect. **Things will go as usual and they will pretty much have to remake/surrender/surrender early regardless.
**
For a great majority of 4v5 matches this will be a quality of life improvement for the victim team. The select few who have a person on their premade have their power cut unexpectedly, etc will see no change from before.
Think about it. What are the chances of a premade player disconnecting knowingly when LP is on the line for him and his friends? Almost zero, right?
What are the chances of one of these premade players disconnecting because of something unexpected? Close to none. And within that close to none chance, the 4v5 would already be considered a loss. See where I'm getting at?
I don't see a big issue here. It is a major improvement on most fronts and nothing will inherently get worse than it was before.
Honestly, if you have a better AFK solution to bring forward, then go right ahead. Otherwise, might as well keep quiet.