If everyone gets a 'best' win streak at some point during the season, then players who are lucky to get their best win streak at the end of the season will fair the better. So comparatively, this idea makes it less competitive overall.
If you like this idea (although it's already pointed out that you do already get a decay at the end of the season). IMO ,the best idea voiced was for players to use their best (or maximum) arena rating gained during the season to compare at the end of the season. It does not penalise players on the top-spot from playing more games and the idea of 'parking' seems pointless which is the thing you're actually trying to solve.
For what it's worth, I think the a good idea is not to have such a drastic season decay where everyone goes back to zero - have just 1 league decay would do the job, and/or have promotion/demotion of 1 league each season, make it more like a league season in football. There is too much emphasis on who plays the MOST games, why do the elite players have to prove themselves each season for the first 20 games or so ?
You'd also get more winners per season - for people that are in bronze league, then will also celebrate the fact they won the bronze league that season and get promotion. Same for silver, gold, etc. More happy campers, and you give the dog a bone.
More players playing against other players of the same ability.
And whatever the current 'leagues' are meant to be in Arena, it's a system is not comparable to any league system I know. Very much a 1 person concoction.