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.

CalculateDescendants(removeit, stage);
    }
    RemoveStaged(stage, false);
    return stage.size();
}
bool CTxMemPool::addUnchecked(const uint256&hash, const CTxMemPoolEntry &entry, bool fCurrentEstimate)
{
    LOCK(cs);
    setEntries setAncestors;
    uint64_t nNoLimit = std::numeric_limits<uint64_t>::max();
    std::string dummy;
    CalculateMemPoolAncestors(entry, setAncestors, nNoLimit, nNoLimit, nNoLimit, nNoLimit, dummy);
    return addUnchecked(hash, entry, setAncestors, fCurrentEstimate);
}
void CTxMemPool::UpdateChild(txiter entry, txiter child, bool add)
{
    setEntries s;
    if (add && mapLinks[entry].children.insert(child).second) {
        cachedInnerUsage += memusage::IncrementalDynamicUsage(s);
    } else if (!add && mapLinks[entry].children.erase(child)) {
        cachedInnerUsage -= memusage::IncrementalDynamicUsage(s);
    }
}
void CTxMemPool::UpdateParent(txiter entry, txiter parent, bool add)