While budget is always a critical issue for the humanoid league, KidSize is currently the humanoid league which is closest to achieving teamplay and, from my point of view. Moving to drop-in only would kill in the egg any chance of producing teamplay. Moreover, this would not solve anything because team who built/bought kid-size robots would have to build larger robots additionally to their kid-size robots, therefore, it will even increase the cost.
An option which is already available this year for team with tight budget constraints is to apply as a joint team, thus reducing the number of robots to be build. And, currently, teams can already apply to the league with a single robot even if this is a strong disadvantage.
For the future, it is likely that there will be a division A/division B scheme in Kid Size. This could help to reduce the number of robots required to reach satisfying performances without requiring 4 robots.
Another option which can be discussed is to start the competition by several round of drop-in games which would serve as a basis to create joint teams when required for the rest of the competition.