What about hidden services? In my opinion that's the real heart of Tor. I think the value of exit nodes will start to wane as Tor gains a larger and larger user base and becomes it's own truly viable darknet. As it stands the majority of security related discussions I've come across regarding Tor strongly discourage spending any large amount of time on clearnet services anyway as this create many potential security/privacy risks.
If you are more interested in the hidden services aspect, did you also have a look at I2P [1]? It is more geared toward this aspect (exit nodes = outproxies in i2p-speak for example are just standard hidden services)
"With that in mind, what user would want to operate an exit node?"
All of the people who have already been running exit nodes for years? Those who don't mind getting that terrible knock on the door? Those who have already had that knock on their door, sometimes more than once, but who live in locations where they don't need to fear their local police?
They can, and they do. Which is why you need to use encryption as well as Tor if you want to something private. But that's the case without Tor anyway.
In theory you could run an exit node with a locked down exit policy, e.g. limited to web traffic which is routed through an IWF compliant proxy, disallow HTTPS and/or payment processing sites, add an HTTP header for reporting abuse/requesting blocks, etc.
Tor's cause is a noble one, but I fear it will end up (if it hasn't already) being used mostly by bad people for bad things.
Once you get your home or server IPs connected to black market forums, drugs, human trafficking or kiddie porn sites and your life (and your family's) has suddenly become more complicated, I wonder how many will still be eager to run Tor exit nodes? Maybe a few dreamers and anarchists.
Tor exits will be shut down one by one, either by the owners or by the police/state.
What we need to come up with is a fully encrypted, anonymous, self-contained super-internet of something like Tor "hidden services" only.
None of the nefarious services and use cases you list require the presence of exit nodes. Silk Road, for instance, is a hidden service. Obviously I don't know about the rest, but you'd have to pretty stupid to put such things on the public internet.
The only people who suffer from the loss of Tor exit nodes are the 'more genuine' or 'more worthy' users who need to use it to escape oppression, or state monitoring.
Exactly, the people who need it "the most" will prolly be left without it at some point.
From the comments I see for a proper "dark net" we now can choose between Tor (without exit nodes) and I2P, this is actually great news.
Not to mention annoyance for the user... there will be sites you can't access since many sites block exit node IPs outright because of abuse.
If some random does something illegal and they happen to be using your node, guess who gets the knock on the door?
With that in mind, what user would want to operate an exit node?