Right - as of recently, I would claim that the market is distorted in a weird way where hiring senior people is just a better deal for most startups than hiring

Right - as of recently, I would claim that the market is distorted in a weird way where hiring senior people is just a better deal for most startups than hiring juniors and training them, even factoring in all the associated costs, because a weird effect where market rates for senior developers are maybe ~2-3x higher but they are 5-10x more effective. Market rates for senior developers relative to juniors should go up in this scenario, but they haven't really, and I don't understand why. The main selfish economic reason to hire junior developers is if the marginal return of hiring someone sooner swamps the relatively higher total cost, which is why you'll see companies like Google that have huge RoI on engineers hiring many more college grads.