Main question (focus on this incase you are looking to close it as "needs focus"): Why can't we change default minimum fee rate to something below 1 sat/vByte?
It was done in past: https://github.com/bitcoin/bitcoin/pull/3305 why not again?
Why does nobody respond to comments in this pull request? https://github.com/bitcoin/bitcoin/pull/13990 Do I need to be from some OG devs group or from blockstream or chaincode labs to contribute and discuss? Or some PRs are intentionally kept pending because a certain group doesn't want things mentioned in it?
Do you think it will result in spamming? I don't think so and few other people as well: https://www.reddit.com/r/Bitcoin/comments/kjrbny/should_default_minimum_fee_rate_be_01_satvbyte_now/ggyym67
The primary reason for the minimum feerate (and related things like the discard feerate) is preventing the use of the P2P network as a cheap global broadcast system. If it's easy to produce transaction that will never confirm yet will relay across the network, it'd open the network to abuse. I don't know what the right level is to make sure that doesn't happen, but 1 sat/vbyte seems pretty low.
That was before the network and mempool logic had floating fees. Now the feerate is primarily set by market pressure, and lowering the minimum would at most have an impact during times when there is ~no demand for block space. I personally think the ecosystem should expect to function fine when there is continuous back pressure, so I'm not sure what is gained by reducing it.
I can't speak for anyone else, but I don't think it's interesting to spend review time on. Note that the PR you link to is just to increase precision of the internal data types used for feerates; it doesn't change any relay policy.
I don't think spam is the problem. Lately the feerate has been significantly higher than 1 sat/vbyte anyway, and long term I hope that whatever the minimum is set to, it's effectively never hit. What matters is the discard feerate, which is not set by the market (it's the "additional" feerate you need to pay to replace a transaction in the mempool through RBF or mempool limiting, in order to make sure there is some cost to relaying the transaction that is now expected to not be mined anymore). That discard feerate is by default set to 1 sat/vbyte as well, which seems low.
Why can't we change default minimum fee rate to something below 1 sat/vByte?
I think I'm reading in sipa's answer the number is simply set arbitrarily low enough so that spam is not a problem.
If that's the reason, I suppose the answer to your question is "we actually can change it".
We could set it to 0.2 sat/vByte like you were writing about, then wait and see if that's still preventing spam.
If that still prevents spam, people would be saving in fees. That's good, pretty good. And if it doesn't disincentivize miners from mining, I suppose it'd be just good enough.
But, would the maintenance effort be worth it?
I suppose a majority of current participants can agree that wouldn't be worth it.
The future is unwritten.