I totally agree Jim. Indeed, once you have played 400 games, the average can no longer increase. This is why we see players with 57-58 as average (and at some point most legends will get this kind of average, it will just take more time for some of them).
Taking into account all games is not good either. Indeed, average should give an idea of the actual level of play of a player. When one reach legend, it is likely that he/she will struggle for a while (it took me 100-150 rounds to start to post good scores and 200-250 rounds to post good scores on a regular basis). So the rounds you played at the beginning are not representative of your skill 200-300 rounds later (my average is 62.45 while my average over my last 100 games is 60.04 today).
So it seems to me that a good compromise would be to use the last 100 games played (only for legend, for other tiers, the system used up to now is not bad to avoid too much sandbagging). 100 games is a big number enough to get an average which makes sense and using the 100 last one means that the average can really fluctuate (going up and down).
Regards