I disagree that anyone else could re-implement it. Once you have a critical mass of listeners, it would become the first port of call for any message you wanted to distribute.
As for policing, I think that would probably be where the money would be made - reputation/quality of the messages being posted.
Suppose twitter had become a giant json firehose, and you could listen to it and, for example, build a jobs listings website from the jobadvert microformatted tweets.
It would end up full of fake job spam, and the listeners might want quality filters. I can imagine that you might pay to validate your identity as not-a-spammer.
How would "Twitter" capture that value and not some third-party platform who and offers those filter services? The value and revenue isn't with Twitter then, so then how does Twitter pay bills? Or am I seeing this in a different light than you do?
As far as I can tell a simple freemium model would have worked. You charge for the firehose and anything high volume, but you give a generous free allowance to keep the grassroots ecosystem healthy. They could have played all the same pricing games that Facebook is now playing, except across a much wider range of channels.
I'm not saying it was a slam dunk, but a lot of talk about this back in 2007/2008 and there was a palpable excitement about Twitter apps. Within a few years the media-company mindset had taken grasp, the valuations and expectations exploded, and now we have boring old Twitter which is still a damn cool thing in its own right, but which is seen as a failure because of a combination of pedestrian vision and mismanaged expectations.
Wouldn't spammers (assuming they're spamming to make money, and therefore likely make more than $0 per spam) also just pay money to be validated in the same way?
As for policing, I think that would probably be where the money would be made - reputation/quality of the messages being posted.