The problem with any TOR-like network is who runs the exit nodes. And why would they? You have idealists, or crazy people, and they are often the same, who don't mind the gendarmes raiding their home every few months, or somehow are oblivious to the fact this will inevitably happen (i.e. morons), or you have people who don't fear police raids because they ARE the police. And not only does one group often mimic the other, but occasionally, people travel back and forth between those two groups.
I'd plainly say "forget about exit nodes". period.
Similarly, who are the people who actually would pay for Internet anonymity? You have your idealist group, but the much larger group is people who pay for anonymity because it's cheaper than paying for a lawyer and more pleasant than doing time in prison. I.e., people doing seriously illegal shit.
Noone in clear state of mind would like to participate in a risky business w/o appropriate reward. Only police, idealists and idiots.
Truly robust anonymity requires anonymity essentially being a standard, so that there is nothing unusual about it at all and you actually have to go out of your way NOT to be anonymous. It needs to be built in at the hardware, firmware level so that people don't even notice it. Once you tailor it as solely a commodity for purchase, the "I have nothing to hide" idiots are going to opt out, and anonymity itself becomes suspicious.
Tor exists for like over 10 years. 1600 relays. ROTFL. Using tor exits to the unencrypted internet is a commonly known bad idea. Anonymity being a standard? Only if half of linux distros ship with it set up by default out of the box - something may have a veeeery slight chance to change. If Windows ships with it - well, things will change - but it's utopistic scenario.
Search engines don't work well with tor/i2p sites - no search=no network. Suppose I want to find some "illegal goods/services" - I bet I'll sooner google it in "normal" network than in some tor/i2p. Who will write search engine that can compete with google, but for tor/i2p?
We have tons of "spam links" in "normal net", it's a problem - but search engines help solving it. Who will do it in tor/i2p&alike?
For example, consider the following use case for a Tor hidden service. You wish to run a service, like a mining pool, non-anonymously but with the location of your hot wallet obfuscated. The reason for this is you are afraid of infrastructure-level attacks like the one against Linode or OVH where the VPS provider was hacked, or attacks where someone co-locates a VM next to yours to use a side-channel attack. By hiding the location of your server, the surface area for attack is shrunk considerably.
A) you already can run it that way for free
b) the hacking approach will be to silently hack (get hired or even get a court order) into VPS provider where you're running your frontend, examine its configs - and voila, the backend is located either somewhere in the normal net or some tor node. Finding the real location of the tor node can be way more difficult of course - but it's rarely needed. I don't think it's a problem to DDoS a tor node/serivce either (surely on app level, not by bandwidth attacks).
As for "why not use browser XXX" - using a browser alone doesn't save your ass. I got somewhat frightned when I got mine almost exact location (that is like 50m+- precision) when pressed "allow to find my location" in a browser. Even if the browser was running via tor - the result would have been the same (sort of a homework - guess why). Using anonymizing proxies - well, it's faster, but who runs them? "the good guys, no logs, bla-blah" - BS, it's not really necessary to log anything as everything may already be logged at upstreams. It's even worse than tor.
