It seems to me that in all the hubbub surrounding the Twitter API changes and the service's shifting priorities from it's users to it's customers that one potential outcome is being overlooked. It is possible that Twitter's attempt to make itself into an exceptionally profitable (as needed to satisfy investors) business may actually not be fruitful. It seems to be a foregone conclusion that Twitter will simply do what is necessary for it's inevitable success. I'm not sure that it is a foregone conclusion.
Whatever the eventual outcome it is clear that the impact on the average user right now is probably pretty insignificant, but if Twitter does struggle to generate significant income then it's plans will become increasingly radical. With no clear indication that Twitter is able to generate significant profit at this point it seems almost certain that the impact on users is going to increase over the next few years. This will grow more disruptive as twitter is in the unfortunate situation that it has fairly limited control over use of the service when compared with the likes of Facebook. Facebook interaction happens through it's own channels on it's own terms and it's clear that Twitter is attempting to wrest similar control from third party apps with it's recent API changes. Twitter feels the need to control your interaction with the service to ensure that all it's user's have a consistent experience of timeline embedded revenue streams.
Loss of groups of users may not be an immediate threat to Twitter. Twitter is colossal in Social Network terms, and users that object to a change in the service will always be an insignificant number compared to the growing pool of users who don't really notice the change. Twitter is highly unlikely to see a large-scale exodus from the platform. That would be unprecedented; most social networks that fail simply never hit critical mass.
There's another group of failures though: the ones that dwindled and are still dying the slow death of user apathy. Services such as MySpace, once enjoying the kind of popularity currently enjoyed by Twitter and Facebook simply faded into irrelevance. Many of it's users likely never made a choice to 'move' to a competing service - they likely just found a newer service to be more personally valuable and that disparity grew over time until the point where interest in the old platform was all gone. This should be Twitter's fear; that in forcing their users to change their use habits they damage the perceived value of the service and allow a plucky new upstart to turn the heads of it's social-media-glutton userbase. It's drive to monetize may cripple it's ability to compete with new paradigms and fresh ideas by holding it's users to the established routine.
Whatever happens, though, it's unlikely that Twitter would die. It's trouble generating cash is also part of it's strength. Anyone in the world with an SMS or internet capable device can take part in the 'global' conversation, and behind the celebrities and shallow trends will always be stories like those that develop during global disasters. Twitter does some good in the world - or at least enables more of the global population to contribute to the greater good than has ever been possible before. A paywall service could never achieve this.