I mean, this is how it always works with kpop companies. No group got where they are on their own, they're always "standing on the shoulders of giants", aka their successful senior label mates. Blackpink got the money for their incredible mv's and producers and stylists and training coaches from Big Bang, 2NE1, and every other YG group that proceeded them. Of course the company isn't just going to recycle 100% of their cut of Blackpink's profits back into Blackpink, unless they're planning to stop being a music company. If they want to keep going into the future long after Blackpink has disbanded, they need to invest in the next generation.
Also, as much as I'm an SM stan, all companies go through highs and lows over the years and a lot of it is just due to luck. New groups and concepts are always a gamble, even the smartest creative directors can't predict exactly what the public will like, so none of these companies can be blamed completely for their current standing compared to the other companies. Of course hard work and smart decisions make success more likely, but never guaranteed.