It's more sustainable than many other business practices. Nobody is worried about sustainability when CEOs are making hundreds of times more money than the average employee at the same company or when shareholders relentlessly enshitify everything so that they can keep getting more and more profit quarter after quarter.
pension plans were not only sustainable since they started in 1875, but the economy thrived and companies grew powerful while workers had them. Historically, some companies tried to screw over their workers and mismanaged their pension funds, resulting in problems down the line, but that was mostly greed.