What is the backup strategy of online-only journals/repositories on the very long term? Disseminating printed documents in many places on the planet is a very robust "backup" strategy for scientific literature. Printed documents (and other forms of ink-on-paper documents), or fragments of them, are known to last for millennia and with a high redundancy (many copies spread in many geographic locations), the chances of being able to reconstruct the original content is high. 

All current electronic data storage require continuous catering (electrical power and servers maintenance) or frequent re-copying (i.e. of laser discs, magnetic hard drives, etc.) to last more than a few decades. 

How do online-only journals and articles repositories intend to ensure very long-term archival of their content?
Most publishers use a preservation network such as Portico or LOCKSS. The principle is that numerous electronic copies are saved, and released in case of certain events (e.g. the publisher stops permanently giving access to the works).