This. They're no longer tied to the employer in hopes of good retirement, they can instead shop around for raises/skills. Which is more money which can be self-invested back into the self-pension/401k. In 401k matching situations, this means more matched, too, instead of an arbitrary salary promise after retirement.

I do not have numbers for comparison of pension vs 401k/self-funded pension. Though I'd love to see what either gets you in similar contributions.