A typical solution to this is to store the data outside of the blockchain and then checkpoint a hash of the data snapshot into the blockchain at regular intervals.
This provides tamper resistance to the data.
Incentivizing users or organizations to store large amounts of data is still a problem of course.
It costs ~$1500 for the Internet Archive to store a TB of data forever. Cut them a check or provide an ongoing endowment (along perhaps with some technical resources to scale up a distributed web instead of existing solely in SF) for them to be storage of last resort (everything else can be a tier of caching on your platform).
Let's Encrypt runs for around $3MM/year. Same budget for Internet Archive allows for adding storage for 2PB of content. Scale up accordingly. No blockchain! [1]
Also ActivityPub relies on servers/federated rather than p2p.