There are a few different ways to interpret this question. First, I should point out that miners *are* required to maintain a full copy of the UTXO set. This only applies to the data that is strictly required in order to ensure that new transactions are valid. When we are referring to non-UTXO-set data, it is possible to prove that the data was in the chain. Somebody just needs to keep the SPV proof and block headers, then anyone with the original copy of the transaction (the person who cared enough to keep it in order to prove that it exists years later, for example), can prove that the data is was the block. But it is not the responsibility of the miners to serve and maintain non-UTXO-set data, it is the responsibility of the creator of such data to ensure that the transactions (and associated SPV proofs) are not lost. Put more sore succinctly, if you are going to reap the benefits of "owning your data", you must also undertake the responsibilities associated with "owning your data." Such responsibilities include maintaining a copy of the data, or paying someone else to do so.
There are a few different ways to interpret this question. First, I should point out that miners *are* required to maintain a full copy of the UTXO set. This only applies to the data that is strictly required in order to ensure that new transactions are valid. When we are referring to non-UTXO-set data, it is possible to prove that the data was in the chain. Somebody just needs to keep the SPV proof and block headers, then anyone with the original copy of the transaction (the person who cared enough to keep it in order to prove that it exists years later, for example), can prove that the data is was the block. But it is not the responsibility of the miners to serve and maintain non-UTXO-set data, it is the responsibility of the creator of such data to ensure that the transactions (and associated SPV proofs) are not lost. Put more sore succinctly, if you are going to reap the benefits of "owning your data", you must also undertake the responsibilities associated with "owning your data." Such responsibilities include maintaining a copy of the data, or paying someone else to do so.