Miners are not required to store OP_RETURN data
Technically miners only need to store transactions that are part of the current UTXO (unspent transaction output) set. Since the OP_RETURN outputs aren’t spendable they are not required to store them in order to validate new transactions.
Those outputs are part of the blockchain, but since nobody is actually required to store them there is no guarantee that they will be generally available in the future. It is up to people that rely on that data to make sure those transactions get permanently stored.
The assumption is that businesses will emerge that will provide long term storage solutions and you will need to pay them for their services. They could charge per read request, you could pay them to store a transaction for a given amount of time, … One obvious candidate are block explorers (as mentioned by Liam Missin of WhatsOnChain in his CoinGeek Seoul talk).