Google Plus is not dying any time soon, contrary to popular belief. Social networks die in one of three main ways - through the creation of a superior network, when creators stop creating due to an inability to garner an audience or when users stop coming due to a lack of content.
With regard to the first point, collections are currently a unique feature possessed by no other network. They provide a significant advantage by allowing you to follow content on a particular niche area much better than when all of a users other posts are thrown in.
Currently there are plenty of creators producing high quality content. This can be seen by visiting featured collections, where, as I have already mentioned, you can choose to follow collections that are only about your particular interests.
Those people who are on Google+ are mainly here to follow people that they don't actually know in real life. This means that you don't get the same level of reverse network effects, with people leaving because all their friends are on Facebook. So Google+ has the ability to persist into the indefinite future and we can expect it to continue to grow as the quality of collections keeps increasing.
With regard to the future, the difficulty will be bringing your everyday people into Google+ and convincing them to browse collections. This might actually happen. It's hard to figure out what features there are, but Google can't be blind to the fact that adding collections has massively increased their community engagement.