It's your subjective view that data isn't needed to be stored on-chain. As governments crack down, as what just happened with Telegram, the need for data to be removed away from a single company or location is a need that SOME desire.
This is the purpose of Nostr, but Nostr relays don't store images, video, audio files. Now can they using Bitcoin? Sure, but that's not programmed into it. Therefore, it requires trust. Trust is centralized.
Further, Nostr relays are centralized as admitted by Fiatjaf on his Cloudflare blog:
https://fiatjaf.com/87a208d9.html
On the other hand, with Arweave, all the gateways are the same with content, but different ISPs and coordinators. This is clearly objectively more decentralized than Nostr's Cloudflare and Hetzner relays.
Further, you make no reference to Nostr or the entire internet's dependence on Cloudflare. I suppose you think it's good that this one company controls and sees all data?
And finally regarding your speculation point,
Bitcoin also has speculation, as does anything with value. You're talking right now using an electronic device, one COULD speculate on electronics. Does this mean there is no value in electronics?