Most people do not have 100 GB of media on hand, nor will they probably in the next 5 years. As a result, it's basically unlimited from a reasonable usage standpoint. Before adding a 100 GB cap, I actually experimented with having "unlimited storage", but this can go wrong in many ways. I've spoken to companies in the VPN and cloud storage industries and having unlimited anything can go very wrong, especially if your service is promoted in a problematic context. e.g. unlimited cloud storage in a data hoarding community. By at least adding a cap, I can calculate a maximum cost for every user to influence my financial decisions and restrict abuse.
As for pay-per-use, no plans for now. Kagi's CEO had a really nice discussion about this on their forum and on HN. Simply put, for the demographic I'm targeting, which is quite broad, the typical user would likely get turned off by a pay-per-use model.
I've also priced Everest lower than other paid services, such as Vimeo and Gyazo, while including extra benefits.