See-also: One More Try (archive).
> But Tim is an idiot.
No. You're being silly; and you're being silly from within the comfort of your own bubble. In any actual discussion of economics Timmy would rip you to shreds. Being Brave calling him an idiot here ought to be beneath you.
> Mr. Connolley
I didn't waste three years of my life for you to call me "Mr".
> A perfectly contestable market is not possible in real life.
Sigh. Yes, of course I read this. I almost wrote something pointing out that of course the theoretical perfection isn't actually met with in real life, but I thought: no, I'm talking to someone intelligent, I won't bother.
Consider physics. Often, physics problems neglect friction; air resistance. They do so because the fundamental features of the problem can be described in this way, without obscuring the essentials in pointless detail. Naturally, if making exact calculations this can be factored in. You know all this. I'm patronising you. You should not deserve this.
As to contestability: Micro$oft probably does have a genuine monopoly, but as I've already said, no-one really gives a toss any more. They are history.
Amazon and Google are I think contestable. It isn't free, but there are no real barriers, at least not compared to the scale of what is required. And of course Google is indeed contested.
Fb might be different due to network effects. But actually it is contested: many young folk head off to Instagram and so on. You're making the typical unimaginative "there is no alternative" when actually there are choices.