Pinterest receives value from the ongoing operation of Instapaper in the form of continued parsing improvements and aggregate information about links on the web, and that value is enough to justify our relatively small operating costs.
I see it as an insurance for us users: as long as a significant amount of users are paying keeping the service as-is is a valid alternative for the owners.
When it becomes free I fear that someone suddenly starts looking at it as a cost center, I mean: all the benefits you mention seems to be possible without operating an end-user service.
Disclaimer: not a paying Instapaper customer, but I am a paying lastpass customer and a paying google docs customer etc etc.
I can't honestly say that the paid-for services I use are more likely to remain available than my non-paid services.
The key factor appears to be a viable business model, but that's impossible to evaluate from the outside (and sometimes from the inside).
free supervision of extractors for web page content (people complain if the scraper doesn't work)
some notion of link quality