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.

{
    *this = other;
}
double
CTxMemPoolEntry::GetPriority(unsigned int currentHeight) const
{
    double deltaPriority = ((double)(currentHeight-entryHeight)*inChainInputValue)/nModSize;
    double dResult = entryPriority + deltaPriority;
    if (dResult < 0) 
        dResult = 0;
    return dResult;
}
void CTxMemPoolEntry::UpdateFeeDelta(int64_t newFeeDelta)
{
    nModFeesWithDescendants += newFeeDelta - feeDelta;
    nModFeesWithAncestors += newFeeDelta - feeDelta;
    feeDelta = newFeeDelta;
}
void CTxMemPoolEntry::UpdateLockPoints(const LockPoints& lp)
{
    lockPoints = lp;
}
size_t CTxMemPoolEntry::GetTxSize() const
{
    return GetVirtualTransactionSize(nTxWeight, sigOpCost);
}
void CTxMemPool::UpdateForDescendants(txiter updateIt, cacheMap &cachedDescendants, const std::set<uint256> &setExclude)