[Greg Maxwell:] For fees to achieve this purpose, there seemingly must be an effective
scarcity of capacity.
The error is in that very first claim. There need not be a scarcity of capacity for the fees to serve their purpose. The fees could, and should, be mandatory and set in the 'consensus rules'.
In fact, scarcity of capacity in a network means lousy service, which means loss of users, which means that the fees will not rise as hoped....
Fortunately Bitcoin is not meant to be a service.
You are interpreting the argument in the wrong way. There needs to be a scarcity in order for a fee market to be sustained. Fees need to adapt to the ever changing reality of the system and could not possibly be forced or determined through any algorithm.