I guess a lot of this depends on whether you consider juniors to be part of your talent pool. Unless you are retaining them, it is just subsidizing industry at large to train them.
I stayed less than a year at my first two jobs. They bet wrong.
You win some, you lose some. Maybe your coworker juniors became seniors and didn't negotiate higher pay.
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This is standard because after a year of experience, a fresh junior dev is worth roughly twice as much on the open market. Most companies won't give a 100% raise because it would upset the apple cart with other employees wanting similar raises. So the junior dev has to move on to get paid what they're worth.