Even before AI have noticed really small to mid size companies not having enough budget to hire junior developers that they know they'd have to train from the ground up. They'd rather take chances on senior developers and/or contractors.
Junior devs will have an easier time finding jobs in positions that operate in niche industries, because they offer onboarding for everyone.
There are firms that take that to heart, and there is indeed a lot of truth in it. A large amount of skills and knowledge just aren't transferable when switching jobs. But I think it's not hard to create more than 20% of the value. And even if it really is 20% of the value, the profit generated from the work might actually be more than the salary gap anyway, and the 1-year growth curve might be faster for a senior than a junior.