You're not being paid to be busy, you're paid to be useful. A man with a tea spoon digging a ditch is worth a lot less than one with a excavator.

Yes, but if I only use the excavator for 5 minutes and coast the remaining 55 minutes, I'm less useful than someone who uses the excavator the whole hour. Now, if I'm particularly skilled at using the excavator compared to my peers, I might get away with working less. If not, working 5 minutes while charging for 60 minutes will eventually get me fired.

Generally, automation does not make us work less because the expectations rise equivalently.