One of the most powerful things about Arweave is that you can pay once, and store files *forever*. For a lot of people, the simplicity of this model is its best feature. There is a lot of content in the world in which the creator wants strong guarantees about permanence, and is relatively cost insensitive.
For example, let's consider my personal blog, kylesamani.com (which happens to be down right now due to DNS issues). I have written a lot of content there, and I would like it to be available well past the point at which I even remember writing about those subjects, as a permanent record in history of how I was thinking at the time. I believe there are a large number of people who feel similarly about their respective content.
If you contrast this to contract-based storage systems - for example Filecoin - the benefits become very clear. Even if you assume that Filecoin is less expensive over a 500 year period, the follow up question remains: how do you actually enter into a 500 year contract? I doubt anyone on the Filecoin network will because that is so far out of scope for what they're expecting. Moreover, because of the unique design of the storage and bandwidth markets, how do you continue to re-enter into new contracts and manage bandwidth over time?
While these questions may be solve-able, they simply add complexity. Arweave, by virtue of it's unique design and different design goals, makes it dramatically easier for developers and customers to reason about.
This has been a common thread that has lead to an explosion if many online services, and I expect something similar will happen with Arweave.