Depends on the value of the asset, probability of it being damaged, and cost of storing the asset. I could see fractionalized NFT’s, for example a piece from the well-known Banksy collection needing to be stored in a vault because there are multiple owners. But an NFT of a vinyl record or non fractionalized assets could be retained by the NFT owner. I’d personally rather take possession of the NFT and asset. Remeber not your key’s not your coins…the same could apply to asset backed NFT’s. This is will be important for bringing real world assets to the blockchain via NFT’s. If you don’t get your question answered here, try the Centrifuge, Altair, or Naos.finance social channels.
If you are developing something for the blockchain you could consider adding a subscription fee mechanism to the smart contract code used to deploy the NFT’s so that the fee takes care of storage costs automatically. The NFT owner could be required to put up some collateral in case they default on the storage fee subscription. Or you could stipulate delivery of the physical asset if the subscription fee hasn’t been paid for a given time period. That’s my 2 sats! Good question.