Twitter may be declining in popularity among teens, but popularity among teens is never what made Twitter important (insofar as it is important), or what gives Twitter its specific edge.
Twitter is able to remain vital for, among other things, two very important reasons: it is text-first, and it is web-first.
Text is scannable, copyable, translatable, accessible, quotable, searchable, screenshot-able, remix-able, parody-able, and on and on in ways that video and images are not.
The web is linkable, ubiquitous, and accessible in ways that mobile apps are not.
Twitter is the easiest way to put text on the web. While it may be a private, closed platform, and may be entirely irrelevant in some near future, its success is a testament to the strengths of text, the web, and the URL.
Any technology that wants to be similarly relevant would do well to embrace these.