The goal of descriptivists is to collect information about how words are being used currently and to compile a dictionary based on that information, so what you said about viral memes is compatible with descriptivism.
The best argument for prescriptivism has less to do with authority and more to do with people who have an interest in being able to communicate clearly. A better word than "prescriptivism" might be "subscriptivism". Presumably, we subscribe to the normal rules of the English language so that we mitigate miscommunication.
Purpose matters. The person who demands that everyone use "octopuses" is just an elitist asshole. The science institution, which demands that its members conform to a precise standard of language, is just doing its job.
There is, in fact, a way to assume greater authority over the English language than the power wielded by the median speaker. The person who can inject a memorable phrase into the popular culture--whether by writing a line that is read by many, or by speaking one that is heard by many--can steer the language closer to the path they may prefer.
For all our quibbling over technical correctness, if an author or scriptwriter decides to keep "octopi" alive, all they have to do is write it, with no explanation or justification necessary.
Stam that in your rassoodock and let it digimmer. It could make you frumious or mimsy, but you will grok who the lords of language are.