The bitstream is an important milestone but it normally takes years of optimization for a codec to really hit peak quality. There are many decisions to be made during the encoding process which have an impact on file size, quality for specific content types, required decoder resources, etc. The first generation implementations often produce files which are notably larger because they're doing the simplest, most conservative encoding process and subsequent tools may have far more sophisticated optimizations.

If it's more familiar, think of it as similar to saying that the x86 or ARM instruction set has been finalized: the first generation will work but over time every component will be optimized to improve certain areas and there will be tailored versions for specific applications (e.g. video codecs used in chat have hard latency requirements while Netflix can afford a LOT of optimization time for a file which will be streamed a billion times).

I agree with all of that. My point is that AV1 is at the point of maturity where we need to sink maximum effort into improving the encoders, and what you're saying seems to support that.