What is a fee market? Does it mean that blocks should be full, esp. after several halvenings? As we all know, Bitcoin had experienced an unprecedented long-lasting congestion period in 2017. This seems quite horrible to some people generally, but it seems that such phenomenon was on the contrary considered as a positive sign by developers, for example:


  Personally, I'm pulling out the champaign that market behaviour is indeed producing activity levels that can pay for security without inflation, and also producing fee paying backlogs needed to stabilize consensus progress as the subsidy declines.


I've heard that is about a strategy called "stable fee market"(might be inaccurate, sorry). So, I'm courious that does that mean the blocks should be kept full in order to keep Bitcoin safe? Is it merely a compensation of declining block subsidy? Is there any deeper reason behind?

Also, I wonder how could Replace-By-Fee take effects?
The fee-market refers to the tension between 


1) users who want to have their transactions confirmed as quickly as possible and ...
2) miners, who can only mined blocks with a fixed confirmation capacity (1mb, 4mWeight).


Naturally, it is in the interest of a miner to construct a block to mine from the mempool which results in the maximum block reward for the miner. 

So when blocks are full, this tension between demand for confirmation and limited supply of block space will be resolved at a certain price. At a given price, there may or not be a miner who is willing to include a given transaction in its next block. If not, one simply needs to observe the (clearing) price at which transactions are being confirmed. 

The fee-market is therefore between confirmation-seekers (users) and confirmation-providers (miners).

If fees rise because blocks are full, certain transaction amounts with Bitcoin will become unfeasible, because the fees in relation to the transaction amount are too high. So the remaining transactions will naturally be of higher amounts which still justify the market fee price. As long as confirmation capacity is limited and fixed, this will remain true.

This also has nothing to do with halvenings. The inflation/subsidy schedule obviously contributes to a miner's reward, but it independent of any transaction demand, and represents an initial mechanism to distribute coins.