You do a wonderful job of ignoring the several more employees stores hire, even for stores that specifically sell games (In-store assistants, cashiers, managers, inventory checkers; if we go for companies that sell to a nationwide market like Valve does, there's also the issue of how they require branches with their own employees, etc.), which, combined with the operation and upkeep of the several buildings that house the stores (again, viewing a nation-wide chain as comparable to Valve's worldwide electronic store), result in far greater costs that need to be covered by 1/5 or 1/10 of what Valve makes per purchase.
The bolded portion strikes me as an admittance that Valve takes an excessive amount of money per game sale compared to physical retailers. They could lower the price of the game if they felt as though enough was enough as far as pricing goes. Sure they have their frequent sales, but those are often on relatively old games, and there's so many games in Steam's library that any given game doesn't go on sale all that often. Furthermore, what would the difference be between Steam sales and retailer sales/bargain bins?
Additionally, your point about not having the cost of publishing is rather moot for the several cases where there's a publisher and Valve involved, such as with Activision or Paradox Interactive. What's more, if a person doesn't want to get involved with publishers at all, there's also the option of true self-distribution, where they can simply put the game somewhere online for people to buy and download, without Valve taking 15-30% of the sale.
You underestimate the scope of the problem. Major new releases, or even the indie games that, as you said, want and need exposure, often get pushed off of the New Releases page in favor of 1990's PC games that simply got dumped as part of a publisher's back catalog. Not just Early Access games are bad, either, but also a notable number of Greenlight games, games that have a publisher that cleared the system before, or, heck, just somehow get onto the store via their own merits. Several people have compared Steam's store to the pre-1983 North American video game market. There's also a perpetual problem with games being mis-categorized because Valve can't be bothered to even label a game themselves, but rather let the community do it, which leaves labeling the genre of a game up to people who don't take the task seriously, think they're pulling a joke with a lot of other people, or simply don't know what a specific genre is ("open world" versus "sandbox" ends up being a big one, if I remember correctly).
