Sadly it is true, MAC GPU's are often the same hardware used in laptops. Also, Apple sprinkle a special kind of awfulness onto their drivers.
I tested this with Quake III (a game from 1999!). My 2011 iMac could not render the game at 120fps, it managed somewhere between 50 and 70. I bootcamp into Windows, and I get a rock solid 120fps.
They certainly are. Look at the current top 10 for Steam; there are exactly two games, Fallout and GTA, that require anything more than the world's crappiest computer to play. I have no clue about the games not represented on Steam (basically Battlefield and the Star Wars edition of Battlefield), but I bet they follow the same pattern.
The Witness has has a peak of less than 5,000 players at one time so far.
The top two games (and basically the only active "pro" games at the moment), which together have about one million players playing at any given time are both carefully tuned to run on toasters according to Valve's hardware survey.
I like Valve's current micro transaction model. You can buy cosmetic items, but that's it. I can't speak for DOTA2, but in CS:GO they don't impact the game negatively like they did with TF2. Trading from free drops also allows kids and poor people to participate in the economy, which allows them to make more money for Valve than begging mom to buy a $60 game would.
