> Is it really true that miners do/should care about that?

De facto, any miner running an unmodified version of bitcoind doesn't
care about anything aside from ancestor fee rate, given that the
BlockAssembler as-written orders transactions for inclusion by
descending ancestor fee-rate and then greedily adds them to the block
template. [0]

If anyone has any indication that there are miners running forks of
bitcoind that change this behavior, I'd be curious to know it.

Along the lines of what AJ wrote, optimal transaction selection is
NP-hard (knapsack problem). Any time that a miner spends deciding how
to assemble the next block is time not spent grinding on the nonce, and
so I'm skeptical that miners in practice are currently doing anything
that isn't fast and simple like the default implementation: sorting
fee-rate in descending order and then greedily packing.

But it would be interesting to hear evidence to the contrary.

---

You can make the argument that transaction selection is just a function
of mempool contents, and so mempool maintenance criteria might be the
thing to look at. Mempool acceptance is gated based on a minimum
feerate[1].  Mempool eviction (when running low on space) happens on
the basis of max(self_feerate, descendant_feerate) [2]. So even in the
mempool we're still talking in terms of fee rates, not absolute fees.

That presents us with the "is/ought" problem: just because the mempool
*is* currently gating only on fee rate doesn't mean that's optimal. But
if the whole point of the mempool is to hold transactions that will be
mined, and if there's good reason that txns are chosen for mining based
on fee rate (it's quick and good enough), then it seems like fee rate
is the approximation that should ultimately prevail for txn
replacement.


[0]:
https://github.com/bitcoin/bitcoin/blob/master/src/node/miner.cpp#L310-L320
[1]:
https://github.com/bitcoin/bitcoin/blob/master/src/txmempool.cpp#L1106
[2]:
https://github.com/bitcoin/bitcoin/blob/master/src/txmempool.cpp#L1138-L1144