You have a point, but there's a few problems with it IMO..
As long as there are relatively few maps, I would agree it could actually be quite nice like you said. Although that "same amount of time to complete all maps" argument isn't very valid though. It assumed all players who would be participating would be there at the start of the season, which isn't the case..
But let's say that season 1 starts with 50 maps. Then later on when there are 50 new maps ready, we'd start season 2 with only those 50 playable, and so on.
That offers the first issue: when would the first 50 maps be playable again? Would we wait for yet another 50 maps for season 3, or would we wait for e.g. 30 maps, and take 20 maps from season 1 or something? Because never having them playable in the future, would be an extremely bad idea.
And then there's the actual problem: What happens when we get, let's say 1000 maps?
I agree with the problem with ET AP being a bit dull due to "too many maps available at a time", but that's a very particularissue. They have extremely low competition and the largest map selection that exists. Defrag has accumulated records over more years than ETRun, with both more players than ET AP has, and more of which are very active as well. Which in turn makes it more interesting to play/compete. Take right now as proof, it's Saturday evening, not one player is currently on any of the ETRun servers, while Defrag has the usual ~10-20 crowd.
But back to the point. We have 1000 maps, and there's a new season. So now what? Should we pick 50 maps and lock the players down to those for a few months? 200? Should we ONLY launch new maps in the season, leaving all the older maps essentially useless? In either case, I don't see a way that would work particularly good.. Either you'd essentially be throwing away all the older maps, or you'd end up limiting the map selection to a subset of the maps. And a subset of 200 maps could still be enough to create that feeling you were talking about in ETRun..
So yeah,, tell me if I'm missing something, but it all does seem a bit "iffy" for a lack of better words.. My best thought would be to do something like simply splitting the map selection in two or something, and try to release each new season with a pack of new maps as well. Meaning you'd be playing 500/1000 maps. Then we get a season change, and you'll have the other 500 maps available + 50 new ones, or something like that, and so on.. Which I guess could be interesting, or? In either case, it's more relevant further down the line when we get > 200 maps or something like that.