the fact is the past few years most of the popular girls are freelancer, and started their career as a freelancer too, the last 3 best actress winners are freelancer, and current top 4 : Aika, Hibiki Otsuki, Saki Hatsumi and Aki Sasaki are freelancer
so I agree with you big studios aren't producing starlet like before, most of them only have short career in AV or failed to reach the top spot
Sakura Mana who won best new actress at the Adult Broadcasting awards, won the best actress in 2015, the DDM awards are only 3 years old, and the best actress award is influenced a lot by sales, of course actresses who put out a lot would always win those, The award of Excellence is what goes to who the fans vote as the most popular, which have had A contract girl, and Aika win it so your concept on that is kinda flawed.
also if you mean DMM top 4, again that's unfair if you release a lot, great chance you sell more then someone who does 1 video a month, Like Sakura Mana is one of the top 5 girls in the industry, Yua Mikami, and Rion who aren't great performers you'd easily say they are more popular then those 4 freelancers, Acky has been a consistent top girl in the industry for almost 20 years and she does 2 videos a month, Tsubomi also does great at two vids a month.