Currently the mempool isn't persisted across restarts. The historical argument against doing this is that it would make transactions immortal. Expiration now addresses that issue.

Without preserving the mempool node behavior changes drastically across restarts which complicates production use (e.g. mining overly small/low fee blocks) as well as development testing. The lack of retention also causes the loss of txn priority modifications made with prioritisetransaction which makes the API less reliable.

{
    strMiscWarning = strMessage;
    LogPrintf("*** %s\n", strMessage);
    uiInterface.ThreadSafeMessageBox(
        userMessage.empty() ? _("Error: A fatal internal error occurred, see debug.log for details") : userMessage,
        "", CClientUIInterface::MSG_ERROR);
    StartShutdown();
    return false;
}
bool AbortNode(CValidationState& state, const std::string& strMessage, const std::string& userMessage="")
{
    AbortNode(strMessage, userMessage);
    return state.Error(strMessage);
}
} 
/**
 * Apply the undo operation of a CTxInUndo to the given chain state.
 * @param undo The undo object.
 * @param view The coins view to which to apply the changes.
 * @param out The out point that corresponds to the tx input.
 * @return True on success.
 */
static bool ApplyTxInUndo(const CTxInUndo& undo, CCoinsViewCache& view, const COutPoint& out)