Practically everyone.

If you were searching for a tutorial, it was a fantastic time saver.

Somewhere between two and three billion

There were even extensions for showing a bar display of the ratio under the thumbnail. So my guess is the answer would be "enough"

You asked a question no one can answer.

Instead of asking impossible questions, I suggest just using a bit of logic. Officially, YouTube removed the like/dislike because they felt people were prejudging videos before viewing them themselves. Unofficially, people speculate they did it to have greater control of what people watch. But in either case, such a change would only make sense if plenty of people were checking the ratio prior to viewing. If no one ever paid attention to it, then there wouldn't be anything to be gained by tampering with it.