It's not as bad as you think. If the frontend and database is open source, the assets will be mirrored, and therefore end up in a giant archive. This is what happened to Hic Et Nunc last month - founder decided to pull the plug, community resumed within a few hours with other sites. The power is in the technology mix: Blockchains and smart contracts secure the transactions, everything else can be secured with existing methods. It's an incentive structure shift that's still being explored - you always want a 100% archival because it protects the represented value on-chain. It pushes the business model away from platform control as well. They will most likely have to differentiate with curation and discovery services.

Currently, a lot of the frontends are not open. They pose the same degree of rugpull risk as any ordinary web site. That is unlikely to remain the status quo since both collectors and artists will ultimately demand a proof of redundancy. But as it stands, it's a bubble, and the rules aren't set.