For every 1 kill there is 1 death. The average overall k/d for the game is 1.0 - the game's "average player" isn't getting better. That being said, the people that are above the average will tend to keep playing while a worse player will be more apt to play less. I'd say that instead of the average player getting better - the top 25% or top 10% are improving. Not to mention as more players come into the game, the number of "10th percentile" players increases proportionally.
This is uncorrelated to the 3rd party aspect though because as more players are left - Fall Skirmish for example - the more "parties" are involved in your fight. So to say more players late-game results in less 3rd parties is untrue.
Finally, I agree that 3rd parties will always be a thing in a BR game, and I 100% agree that the mobility items they are adding are great for re-positioning and resetting a fight. That was one of the best arguments for the Grappler that I've read on here.