decent, free development tools
Well, you can have these for any mature platform. Be it QT, Flex or even Java. I'm not sure what's more decent about the HTML toolchain. The often cited FireBug is, to me, merely an emergency bandaid that makes development at least possible but still far from enjoyable.
the ability to interpose proxies and application middleware between the client and server transparently
There's nothing stopping any fat-client to speak HTTP. You don't need to a browser for that.
and a very, very large install base for the client runtime.
I'd say installation base is a red herring when talking about cross-platform apps but I'll give you that "instant-on" is an argument. For that I'd point to Air, which can run hybrid in and outside the browser.
I'm not arguing that HTML doesn't have it's place, mind you. But the extent to which it is abused nowadays is absurd.
You are underetimating this. For evrything else, (air, java, Qt, C#) I have to start a new app. My firefox is always on.
Java too has a very large install base. It is a shame that SUN didn't do much to promote it but you can still take advantage of the fact that many many people have the JVM installed.
That being said, I share much of your amusement at/disdain for the attitude of "OMG, look at this brand new thing I have made that is completely different from all those old things!"