The odds are visible to everyone so we are already aware of what we can potentially win, why worry about how much the bookmaker earns?
As if we go to the pizzeria and pay for a pizza $6.90 the $0.90 is their profit with the $6 they pay us everything else, but we are aware of what we are paying and why on earth should we care about how much they earn?
This discussion is really very strange, rather I wouldn't worry so much about the bookmaker's share or earnings, but about his reliability, his reputation, the speed of payments in the event of a win.
Furthermore, the quotas are assigned not only by professionals who study, but also by users' choices (it may seem strange but it is).
I am the kind of person who believes that it is not actually our business to know how much a company makes.
As long as they are competitive and fair with odds, when people will continue to flock to bookies in the long term.
It is not a secret that, if some bookie started to offer very unfair odds in order to get more money from bettors, then people will find other places to deposit their money.
Those are basic dynamics of free markets, and fair competition.
It would be very different if a single casino hold a complete monopoly over sport betting though.